Okay...here we go.
Step 1:
First, you'll want a picture of a 'Mech against a green screen type map. (Pinkmap in my case, which, I dunno where to get it from, so you'll probably use Big Blue (a blue map)).
Step 2:
After loading the image into your favorite photo/image editor (Photoshop for me.), you'll want to crop the image down to a small box around the 'mech. Taking the crop tool, create a small box that contains the mech.
Step 3:
Now, grab the Magic Wand tool, and click into the pink area around the 'mech. This should select the majority of the pink around your Loki. Should it not, then increase the Tolerance on the tool properties and try again. You want to get most of the pink, but not any of the 'mech.
After the pink has been selected, hit your Delete key, and away it goes. Now, you'll notice little 'holes' in the 'mech where pink shows through. That's okay, click on those and delete them as well.
Now, there may be slight pink jaggies surrounding the 'mech, but again, that's okay. I usually go and make a new layer below the Loki, and fill it with white (or a color different from those of the jaggies, one that will make them stand out.). I then take a 1-px square eraser, and delete the jaggies one by one.
Finished:
That should be about what you end up with (with the checker board being transparent).
Woo! Anyway, there are other ways to do it...I personally find that the easiest, but others may do it other ways. If you've got a screenshot you just absoloutley love, but it's not on a green-screen map, that's okay too....just erase a 1-px border around the 'mech, then grab the magic wand, make the tolerance 255, then click the area outside of the 1-px empty border, then hit delete.

Weeee!
If you have