What you need to do is add an Alpha Channel. This is found in GIMP or probably anything similar. Go to your preset's layer and right click it. There will be a list of actions you can do. Just look for the Add Alpha Channel button and click that. Then, go over the white stuff and erase it. Once erased you should see a light and dark grey checkered background. This should let you know that it is going to be seen as transparent on those spots. Also, make sure you export it as a .png, if placed as .jpg, it may not show up as transparent. If you need screenies I would be happy to oblige. ^^
(Just now notices that you have figured out the problem. *Scoots away like nothing ever happened*)
