in console:
bind "." "devshots_screenshot"
render your superdof, click the 5, press .
it'll save an uncompressed .tga file in your screenshots folder. open this in paint.net or whatever and save as a .png. sign up for imgur and upload the PNG there. boom, uncompressed gmod image.
you can also do jpeg_quality 120 but as soon as you upload a jpg anywhere the quality is murdered