What is it?
Gimp is image manipulation software. It has extensive features for manipulating and creating images, and is probably the standard software for Linux users. There are also versions for other platforms though, so don't let that stop you.
How to support
The Gimp website has a special donation page listing the donation options. They accept donations through the GNOME project, and you can use Paypal, Flattr, Bitcoin, Wire Transfer and Cheque to donate.