They are listed first by file type (.jpg, .png, .svg etc), and then by priority.
This page should be especially watched by the few users that can upload .svgs.
How to upload .svg images ?
.svg images are vector graphics using an open-source format. Unlike raster graphics, vector graphics files are actually text files (based on .xml) which can be rendered as an image, using features not unlike those of html/css. .svg being a W3C standard, they can be seen using standard-compliant web-browsers, such as Firefox or Opera. Microsoft Internet Explorer users have to download an adobe plugin to be able to see those images in their browsers.
Besides of that, every user can simply download an .svg image using their browser's “download the link target as” feature, knowing, for Wikipedia images, that the image on its own page is a .png version of the .svg image, but also a link to the true .svg file. Editing .svg images can be done using various software, one of the best ones being undoubtedly Inkscape, a free open-source software.