I use Paint.NET, and it works great. It is absolutely free! It can be downloaded
here.
In the past I tried downloading other programs and I would just get a limited feature version that would last only 30 days. With Paint.Net, you get ALL of the features and it's completely free for as long as you use it.
If you're not used to layers, it can get frustrating. The Help toolbar really helps you out.
-Soccer