By default, hitting "Ctrl-Alt-Shift-S" will save a screenshot of the sim to [documents]\iRacing\screenshots\ folder. 


In your renderdx11.ini Make sure these are set to 0

CompressTexturesSuits=0 ; 0=uncompressed 1=block compress (recommended)
CompressTexturesHelmets=0 ; 0=uncompressed 1=block compress (recommended)
CompressTexturesCars=0 ; 0=uncompressed (warning! no!!) 1=block compress (highly recommended)


and in the app.ini you can adjust the file type it saves as