So Steam, by default, when asked to take a screenshot will merrily scatter them hither and yon across your hard-drive with unwarranted abandon. Specifically, it’ll put them in the app’s home directory. This isn’t great, because what with cloud-saves and game streaming, I tend to treat the hard-drive with my games on it as transient, and not backed up.
However, Steam also has developed a setting called “Save uncompressed screenshots”, and if you set that and a directory, it’ll put all your screenshots in one place! Hurrah!
Except now all your screenshots are called things like “306760_20160825142347_1.png”. Now, the bit before the underscore is a Steam app ID, so you can look up the game name and file things nicely into the right folders.