It's easy to upload screenshots.
1. Make sure your screenshots have unique names. Not Sshot001.jpg or anything like that. Name it after your map if it's a screenshot of your map.
2. Go to a thread you want to add your screenshots to.
3. Start a new reply. Do NOT use quick reply, you won't be able to add attachments that way.
4. Scroll down a bit and click "Additional Options..."
5. Click "Browse..." and select your screenshot.
6. If you want to attach multiple screenshots, click more attachments and repeat 5.
7. POST! (Make sure you fill out everything else first though.)
And just in case you can't find these screenshots, they are all located in your Paintball folder under "Paintball2/pball/screenshots". To take a screenshot in game, the default button is F12.