Hi,
there are several routes you could take. The most easy route, which I understand you have explored to an extend, is making your own images with a paint program or a photo-editor. The ease and succes of this method are greatly dependend on your skills and the mastery of the software you use. You've been using GIMP, but you might also give these (free) programs a look, that have different interfaces:
-
Sumopaint
-
Artweaver
-
MyPaint
Another route is taking a picture that is free to use in an commercial project and then editing it so it fits the style you are persuing in your program. I've googled a bit for you and found a few blog-posts that sum up a number of image-sites with stuff that is free to use. I haven't checked all the links, but you should be able to find quite a lot of material:
-
blog that lists 10 free sites
-
blog that lists 16 free sites
-
blog that lists 13 free sites
So in combination with the tools listed above, you could find a jungle or underwater-scene and paint it over to your liking. Some software can even paint images for you to an extend. Not sure if the programs I listed can do this, since I use Corel Painter and Art Rage Pro, which are paid programs. (I suck at art though, so I've been focusing on coding for a while). Anyway, hope this helps.