How do I see screenshots taken through Steam of a non-Steam game?
Solution 1:
Right click on the game name in your library and choose 'view screenshots'.
Steam doesn't know what to do with non-Steam games other than run them, and use the overlay on them (sometimes), so it doesn't show any images or other data about the game in the library.
To add these screenshots (or any others) to your profile page online, select the image in the 'screenshot viewer' using the step above, and then click the 'upload' button on the right.