It looks like all the "locked" characters shown on the characters page all have a number associated with them, and you can just type the squad.png to get theirs. But if you look at those numbers, you can see that they do not go 1-100. It goes something like 1-9, 110-119, etc. Go to the characters page in Firefox and look at the Media tab of the Page Info window, you can see these numbers. Otherwise, there isn't going to be any (legal) programs that are going to find images that aren't linked off an existing page already. You should make a post in their forum maybe?