So I've recently been getting into program skinning/GUI modification, and I've been doing some modifications to MANNiEmedic's Antar theme, both for my personal aesthetic taste and to match AlbinoAsian's Myriad suite of skins. However, a continuing problem for me is that when I edit the system image files and replace them in the system32 .dll files and in the .msstyles file, they seem to have trouble with transparency, and have very ugly white boxes around them. As an example, here's the start button. Here's what I'm trying to replace my current button with:

It's not perfect yet, but I'm still working on it. I created this image by just drawing over the original in Photoshop and resaving it, without altering the dimensions or anything else. As you can see, it's a transparent png, just like the original. However, when I replace the system file, this is the result:

and here, when I press the button, the white border completes itself.

I'm rather new to this whole thing and think I'm getting along all right, but this transparency thing baffles me. Is this a common problem? How can it be fixed?