I cheated? haha I sized the original screen cap down, then pasted it as a layer onto the larger Spike screen cap, added a vector mask to the pasted layer and went nutty with a brush. It's erasing, but easier to deal with oopsies than with the regular eraser.

I get so impatient with the lasso tool. :\ I'll have to try the magic wand trick, though, and use the lasso on smaller bits, maybe. Thank you! :)