try planetrenders.net its free to make an account and when you do you can just search for images, and when you find one you like, open it, then click it to open it full screen, then right click>view image, save the image, you have your render in your renders folder. woohoo. but honestly, those bg's still need a lot of work. i would call my bg's only decent. you should see some of the people in the critique topic. there is a lot of flow, composition, lighting, feel, and more that goes into the bg.
rendering's quite a bit easier in PS, but in gimp, just remember to zoom in to around 400% and use polygonal lasso. then just click a bunch with every little curve to follow curves without it looking liney. before you make the selection set the feather to 5 px. once you select the render go to select>invert selection. now press ctrl+x (cut)