The best thing to do is make the background of sig images transparent because we have a lot of different skin options for SWC that all use different backgrounds, selectable under Settings -> Look & Feel. Forum post background is black for me so it would be #000000, but yours may not be. You can paste a screenshot into your image editing program to check what colour your own background is, but transparency is the only way it'll look best for everyone rather than only on your screen.
(Incidentally, some of the skins are currently being edited for improvements, so you might want to wait until next sync when they are synced over for usage.)
If you don't have anything other than Windows 7 Paint, Paint.Net is a lightweight, free image-editing software that will let you make the background transparent, and save it as a PNG to keep the background transparent. Other people use GIMP or Adobe Photoshop. I find Paint.Net is just fine for basically everything I need to do myself art-wise in SWC.