The GIF image format, "Graphics Interchange Format", is an old image format designed in the era of graphics cards handling 16 to 256 colors. It utilizes very simplistic compression, and has very limited support for transparency.
The modern alternative to the GIF format is the PNG format, which while also being (near-)lossless, handles true color display and a separate "alpha" (transparency) channel that allows for smooth edges (anti-aliasing) regardless of background color.
Examples
These are examples of a GIF versus a PNG image with different background colors:
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If you are viewing this with Internet Explorer prior to version 7, you will see a dark gray box around the PNG icon with the white background. This is because prior to version 7, Internet Explorer does not fully support PNG transparency; it will shade toward the "background color" element of the image. This icon has #333333 (dark gray) set as its background color to work well with the default WoWWiki skin, which all unregistered visitors use.
