Yeah. Do what Chrono Said. Im only speaking from what I know of GIMP, so tell me if There are any discrepancies in my explanation:
What you need to do is,as Chrono Said, adjust the Tolerance of the magic wand (It's Called "Threshold" in GIMP land) so that it will select the background color range, but not the actual future-render. The 'paths tool' (if an option in PS) would certainly be one's most favored ally in this, and if nothing else, one could use the "Smart Scissors" (another Questionable item) and just select various in-line points on the outside of the render, making sure to adjust the paths in between the selection points. Then, one can just invert the selection or cut and paste into a new file.