Promoting your wiki means you're putting it on www.wikia.com. The front page of Wikia. Look at that page: still images and more still images. A video wouldn't work without a serious redesign of the site
A PNG image is desirable because of its lossless compression, which in extremely simplified terms means that basically it's less likely to look bad... though honestly requiring it be PNG to upload is a bit silly. In any case, converting to PNG should not be terribly hard in any image editor... that includes MS Paint, mind you. Just open the file, and "Save As" a PNG image.