You will first need to enable this within the sim by going to Options>MISC,  then check the box that says Enable video and screen capture.

Log out of the sim and back in, then, by default, hitting "Ctrl-Alt-Shift-S" will save a screenshot of the sim to [documents]\iRacing\screenshots\ folder. 

If you are unable to save the screenshot, 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