the bottom left pixel of the image determines the transparency color, chances are that the outline is using that pixel, which in turn, makes the outline transparent.
Thanks. That helped. The only other thing I should point out involves the music used by specific skins. Sometimes, the loops tend to be broken and thus become imperfect. I have this skin called (2.0) Tweaked Super Mario World Skin, which I would assume would be for the outdated base 2.0 version, since I'm currently using 2.0.6B, like I said. Here's the link to said skin: