I love TigerLaunch! It looks like you should be able to edit the icons in Iconographer easily enough. You might have to name yours the same as one that's in there and replace it. They are in the resources folder inside the app. You look inside the app by ctrl clicking it and choosing Show Package Contents from the contextual menu that pops up. You probably can't get rid of the shadow though except with Unsanity's ShadowKiller or WindowShade, which will get rid of most of it, along with all other window shadows too!

If you make a really good icon, post it here because I don't really like any of the stock ones that well either. You do know that it comes with 13 different ones don't you?