Is it possible to take Steam screenshots without using the in-game overlay?

I've tried to automate the process described by m0nde. It requires PowerShell to run, but if you have Windows 7 or newer you have it preinstalled.

Save this as run.bat:

@powershell -NoProfile -ExecutionPolicy Bypass -File main.ps1

And this as main.ps1:

Function Get-VDFContent ([string] $path)
{
  $fileContent = Get-Content $path
  $obj = @{}
  $stack = New-Object System.Collections.Stack

  $group = [regex] '^\s*"([^"]+)"\s*$'
  $keyVal = [regex] '^\s*"([^"]+)"\s*"([^"]+)"\s*$'
  $bracket = $False

  ForEach ($line in $fileContent)
  {
    If ($bracket)
    {
      If ($line -Like "*{*")
      {
        $bracket = $False
      }
    }
    ElseIf (($match = $group.Match($line)) -And $match.Success)
    {
      $obj.($match.Groups[1].Value) = @{}
      $stack.Push($obj)
      $obj = $obj.($match.Groups[1].Value)
      $bracket = $True
    }
    ElseIf (($match = $keyVal.Match($line)) -And $match.Success)
    {
      $obj.($match.Groups[1].Value) = $match.Groups[2].Value
    }
    ElseIf ($line -Like "*}*")
    {
      $obj = $stack.Pop()
    }
    Else
    {
      Throw
    }
  }

  Return $obj
}

Function Create-ScreenshotPath([string] $screenshots, [string] $date, [string] $i)
{
  Return Join-Path $screenshots ($date + ($i.PadLeft(5, "0")) + ".jpg")
}

$steamPath = ""
If (Test-Path "HKCU:\Software\Valve\Steam")
{
  $steamPath = (Get-ItemProperty "HKCU:\Software\Valve\Steam").SteamPath
}
If (-Not $steamPath)
{
  $steamPath = Read-Host 'Enter Steam install folder path (example: "c:/program files (x86)/steam")'
}

$loginUsers = Join-Path $steamPath "config/loginusers.vdf"
$users = (Get-VDFContent $loginUsers).users
$lastUser = ($users.GetEnumerator() | Sort-Object { [int]$_.Value.Timestamp } -Descending)[0].Name
$lastUserShort = $lastUser - 0x110000100000000

$userPath = Join-Path $steamPath ("userdata/" + $lastUserShort)
$localConfig = Join-Path $userPath "/config/localconfig.vdf"
$apps = (Get-VDFContent $localConfig).UserLocalConfigStore.Software.Valve.Steam.apps
$lastPlayed = ($apps.GetEnumerator() | Sort-Object { [int]$_.Value.LastPlayed } -Descending)[0].Name

$screenshots = Join-Path $userPath ("760/remote/" + $lastPlayed + "/screenshots")
$thumbnails = Join-Path $screenshots "thumbnails"
New-Item -Force -ItemType directory -Path $thumbnails >$null

$scriptPath = Split-Path -parent $MyInvocation.MyCommand.Definition
$date = Get-Date -Format yyyy-MM-dd_
$i = 1

While (Test-Path (Create-ScreenshotPath $screenshots $date $i)) { $i++ }

$filesToMove = Get-ChildItem $scriptPath -Filter "*.jpg" | % { $_.FullName }

ForEach ($file in $filesToMove)
{
  Move-Item $file (Create-ScreenshotPath $screenshots $date $i)
  $i++
}

Now put those files in one directory with screenshots you want to upload and launch run.bat. This script finds last logged-in user and last played game so keep it in mind before running.

A small modification to Piotr Kowalski's answer. Now it also makes thumbnails for your screenshots.

Take the file from this gist or just paste the code into your main.ps1

file:

# Modified version of @piotr-kowalski's script 
# Now also makes thumbnails for screenshots
# https://gaming.stackexchange.com/a/238288/212957
#
# Note:
# This script finds last logged-in user and last played game so keep it in mind before running.
#
# Usage:
# 1) Save this as run.bat:
# @powershell -NoProfile -ExecutionPolicy Bypass -File main.ps1
# 2) Download this file
# 3) Put both files in one directory with screenshots you want to upload and launch run.bat.

Function Get-VDFContent ([string] $path)
{
  $fileContent = Get-Content $path
  $obj = @{}
  $stack = New-Object System.Collections.Stack

  $group = [regex] '^\s*"([^"]+)"\s*$'
  $keyVal = [regex] '^\s*"([^"]+)"\s*"([^"]+)"\s*$'
  $bracket = $False

  ForEach ($line in $fileContent)
  {
    If ($bracket)
    {
      If ($line -Like "*{*")
      {
        $bracket = $False
      }
    }
    ElseIf (($match = $group.Match($line)) -And $match.Success)
    {
      $obj.($match.Groups[1].Value) = @{}
      $stack.Push($obj)
      $obj = $obj.($match.Groups[1].Value)
      $bracket = $True
    }
    ElseIf (($match = $keyVal.Match($line)) -And $match.Success)
    {
      $obj.($match.Groups[1].Value) = $match.Groups[2].Value
    }
    ElseIf ($line -Like "*}*")
    {
      $obj = $stack.Pop()
    }
    Else
    {
      Throw
    }
  }

  Return $obj
}

Function Create-ScreenshotPath([string] $screenshots, [string] $date, [string] $i)
{
  Return Join-Path $screenshots ($date + ($i.PadLeft(5, "0")) + ".jpg")
}

Function Save-Thumbnail([string] $imagePath, [string] $pathToSave)
{
  $wia = New-Object -com wia.imagefile
  $wia.LoadFile($imagePath)
  $wip = New-Object -ComObject wia.imageprocess
  $scale = $wip.FilterInfos.Item("Scale").FilterId                    
  $wip.Filters.Add($scale)
  $wip.Filters[1].Properties("MaximumWidth") = 200
  $wip.Filters[1].Properties("MaximumHeight") = 150
  #aspect ratio should be set as false if you want the pics in exact size 
  $wip.Filters[1].Properties("PreserveAspectRatio") = $true
  $wip.Apply($wia) 
  $newimg = $wip.Apply($wia)
  $newimg.SaveFile($pathToSave)
}

$steamPath = ""
If (Test-Path "HKCU:\Software\Valve\Steam")
{
  $steamPath = (Get-ItemProperty "HKCU:\Software\Valve\Steam").SteamPath
}
If (-Not $steamPath)
{
  $steamPath = Read-Host 'Enter Steam install folder path (example: "c:/program files (x86)/steam")'
}

$loginUsers = Join-Path $steamPath "config/loginusers.vdf"
$users = (Get-VDFContent $loginUsers).users
$lastUser = ($users.GetEnumerator() | Sort-Object { [int]$_.Value.Timestamp } -Descending)[0].Name
$lastUserShort = $lastUser - 0x110000100000000

$userPath = Join-Path $steamPath ("userdata/" + $lastUserShort)
$localConfig = Join-Path $userPath "/config/localconfig.vdf"
$apps = (Get-VDFContent $localConfig).UserLocalConfigStore.Software.Valve.Steam.apps
$lastPlayed = ($apps.GetEnumerator() | Sort-Object { [int]$_.Value.LastPlayed } -Descending)[0].Name

$screenshots = Join-Path $userPath ("760/remote/" + $lastPlayed + "/screenshots")
$thumbnails = Join-Path $screenshots "thumbnails"
New-Item -Force -ItemType directory -Path $thumbnails >$null

$scriptPath = Split-Path -parent $MyInvocation.MyCommand.Definition
$date = Get-Date -Format yyyy-MM-dd_
$i = 1

While (Test-Path (Create-ScreenshotPath $screenshots $date $i)) { $i++ }

$filesToMove = Get-ChildItem $scriptPath -Filter "*.jpg" | % { $_.FullName }

ForEach ($file in $filesToMove)
{
  $thumbnailPath = (Create-ScreenshotPath $thumbnails $date $i)
  (Save-Thumbnail $file $thumbnailPath)

  Move-Item $file (Create-ScreenshotPath $screenshots $date $i)
  $i++
}

Use SteaScree. It can upload screenshots from any source to the Steam cloud. This includes sources such as a game's built-in screenshot functionality, or other game screenshot software (e.g. Bandicam, FRAPS, Geforce Experience / Nvidia Ansel, etc.), bypassing the need for the Steam overlay.

SteaScree is a simple cross-platform open-source utility tool, which greatly simplifies the uploading of screenshots to the Steam Cloud, which were taken without the use of Steam's in-game overlay. You just pick pics, select game and SteaScree will do the rest.

The problem: every Steam user has 20 GB of cloud space specifically for a screenshot storing. But not every screenshot can be easily uploaded to the Steam Cloud, since the files should have been created by Steam in-game overlay, thus have a specific filename, reside in specific Steam directories and be registered within a special screenshots.vdf file. Steam fails to upload custom screenshots returning "Steam Cloud may be temporarily unavailable" error. SteaScree resolves this and automates the whole screenshot uploading preparation process.