User blog comment:Dopp/Technical Update: March 29, 2011/@comment-3104099-20110401100716/@comment-26686429-20110401104621

Not sure about the speficis (I'd guess the software was written so that it expects .png in some way, and produces an error if it doesn't get it) - but .gif has a rather limited colour pallette (images look bad, due to a lot of loss of colour information). PNG is the logical choice if one format has to be chosen, as it's normally smaller and can store a lot more colours, so if Wikia did need to force one format to make the software work, they'd choose PNG over GIF.
 *  Random Time  10:46, April 1, 2011 (UTC)