There's an issue with Steam Screenshots right now, Screenshots save as TGA files and cannot be uploaded. This seems to happen with every game, But I thought it would be a good idea to share this.
Step 1: Going to Settings
[img]http://cdn.akamai.steamstatic.com/steamcommunity/public/images/clans/12199717/2179c010dfc4ecbd1410904a96ab150d189568a5.png[/img]
Step 2: Pressing the change button under "Steam Beta Update" and changing it to "Opt out of all beta programs"
[img]http://cdn.akamai.steamstatic.com/steamcommunity/public/images/clans/12199717/2cbd38e4cbf9232b4ac2375084b4d07c5b20135f.png[/img]
[img]http://cdn.akamai.steamstatic.com/steamcommunity/public/images/clans/12199717/3e9fa8e6b2e342be1f172770b9f2370530116181.png[/img]
This should've fixed the issue. If you are already opted out of the beta, You shouldn't have this issue.