Posted 27 September 2012 - 18:35
Make sure you're not trying to upload screenshots immediately after exiting a game, as when Steam uploads saved content via Steam Cloud any screenshots you upload will fail - you'll be able to see this as the game names turns yellow in the Steam library and should tell you how much is left to upload. Unfortunately Steam doesn't try to upload screenshots again automatically, so you have to wait for a few minutes and then try again manually.
Screenshots will also fail if your connection to the Steam Community is lost and it's been a bit more unreliable since the release of Borderlands 2. There was quite a large outage the other day. Check your friends list to see if you're still connected.