Alaska Metropolitan of OPIUM Everyday did a blog post with a basic howto on editing ones SL fashion screenshots. Her tutorial uses Photoshop but the techniques were quite applicable to the image editor I use, the GIMP (Gnu Image Manipulation Program).
I was able to take an image of my avatar, select just my avatar out of it and add it to a new bacground created with the paint can, a dollop of a lighter color and gaussian blur.
I had always wondered how people were able to make collages and stuff and select just the bits they wanted and now I know
"And knowing is half the battle"