User blog comment:Harbumz/Introducing Vignette, Wikia's New Thumbnailer/@comment-34115-20150212010632/@comment-31845054-20150212021425

SVGs were tricky, and were the source of many issues. There are a few rules surrounding SVGs specifically, but the gist of it is: when upscaling, we use the density flag when converting images; the resulting thumbnail is in png format.