>>5941591When it's this simple you just select background with the magic wand tool (it's called fuzzy selection tool in GIMP) and delete. Or work with the color selection from the edit menu. I'm not very familiar with GIMP, but it can't be too different from Photoshop.
http://graphicdesign.stackexchange.com/questions/5446/making-the-background-of-an-image-transparent-in-gimp