Have you got any old photos that need retouching? If you don't want to use expensive and hard to use applications then you should try Image Analyzer. It might not be open source but it is high quality freeware that runs on Windows and on Linux with Wine. It has some great features like texture synthesis which will cover holes and other defects of old photos. Just keep in mind that it is not a drawing program, it is clearly a photo editing program focused on having many different filters.
But apart from enhancing photographs you can also do more funny things with it. Take a look at this cow which I made using the texture synthesis tool of Image Analyzer:
Image Analyzer can also be extended with plugins, so if you are a developer you might consider writing one for it. Currently there are about ten plugins.
The only bad thing about Image Analyzer is that it has no documentation. There is a short tutorial on its website and a nice support forum though.
I hope that it gets open source soon... ;-) (some older versions are already open source)