Dood....lay off the pro plus...
I think what Tristan is trying to say is check stuff like the quality slider in whatever image software you're using, jpg compression is a bitch for killing the quality of an image. If you didn't get the decal from somewhere like brandsoftheworld.com and it isn't at least 400pixels square to start with then find one that is and resize it to fit, or make it yourself, otherwise it will look crappy.
Try skinning at 2048 resolution and shrinking your skin down to 1024 when you're done, this won't actually improve the quality, but it will create the illusion that it has.
Avoid pinstripe thin lines if possible, the current version of the game renderer will brutalise them and ruin the appearance of your skin from close up, even though it'll look fine in the cmx viewer.
Finally the compression in the game is more severe that in the CMX viewer, and if you really want to know what your skin's gonna look like, get an ingame shot, and don't rely on the viewer alone.