I've seen a couple of posts asking for a decent, simple image editing tool. I think I've got a good suggestion for a tool that comes in between GIMP and Microsoft Paint.
It's a project that started in a University programming class and has blossomed into a surprisingly powerful image editing tool that is still simple enough for most users. Moreover, it's distributed on the MIT License, so it -is- OSS, which meets the requirements for here. Better still, the install size (at least on a local hard disk) is just shy of 9 MB, so it's a very small, tight application to boot.
I tried loading it up on my PA drive just a few minutes ago and it would need a little fixing up (there are four utility .exe files in the root folder, so Paint.NET shows up five times in the PA menu) but considering the full source is available, it shouldn't take too much work to do.
I'm going to be suggesting the same idea (A PA version of Paint.NET) on their forums as well.