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
Step 2: Pressing the change button under "Steam Beta Update" and changing it to "Opt out of all beta programs"
This should've fixed the issue. If you are already opted out of the beta, You shouldn't have this issue.