Depends where you're getting these images from. If you don't want anything but the picture, simply go to PlanetRenders.net, search for your image, click on the thumbnail (you must do this 1st), and finally save the image. When you open it the background should be transparent.
F.Y.I. You must be a member of PlanetRenders before you can do ^^.
If you have an image you can't find any where but Google, just render it. With the Polygon Lasso tool.