(Try Project Wonderful ads? Adsense seems to be better if you have a larger audience but not very good otherwise.)
I thought of volunteering even though I'm not a GIMP artist, but I genuinely don't think "better backgrounds" is what you need, since you have a gag-a-day comic your readership isn't going to care if the backdrops are photoreal. You can get away with gradients and even flat colors. Hate to break it to you but it's going to be the writing that'll make or break your style of comic.
And I don't think you're that bad at background art. In fact, it meshes very nicely with your character art. I think more detailed backgrounds would clash. What you ought to do is tighten up your shading. Don't use the smudge tool to shade. Use a lower opacity brush instead if you don't have a tablet.
Though, if you ever need any pretty landscapes feel free to hit me up. I specialize in exteriors and landscapes and things. Not really the sort of background you're looking for, I think.