we support png and jpg images. The recommended way to use images in mobile games is by using sprite sheets, because they have a far superior performance as they can be rendered with a single draw call. We fully support the best tool for this job, TexturePacker, which has a great explanation of what sprite sheets are here: http://www.codeandweb.com/what-is-a-sprite-sheet
As vector images are highly imperformant, we do not support them in Felgo. Instead, use a technique called content scaling which is described in a tutorial here: http://felgo.com/doc/vplay-different-screen-sizes.html. It allows your graphics to be crisp on all resolutions, use the least amount of memory and can be rendered with high performance.