I use the free version of Jing for my screen captures but before I found Jing, I would open my screen shot in Paint.NET (it's free as well) and save the screen shot in an acceptable format for wherever I was wanting to upload it. Jing saves them automatically as PNG files so you can pretty much upload them anywhere.
Jing - http://www.techsmith.com/jing/free/
Paint. NET - http://www.dotpdn.com/downloads/pdn.html - it's like a free version of photoshop. It works if you don't have photoshop or illustrator. I have both now, but I still use PDN from time to time
at 4:20 PM on Mar. 21, 2011