Sticker Mule accepts all sorts of file formats: https://www.stickermule.com/support/file-formatsWhich graphics programs are people using to do the sticker art? Does sticker mule have a required format?
I think most designers use Adobe Illustrator, which produces .ai by default, which is a vector format. I use Inkscape (free software), which produces .svg by default, another vector format. Most can also export/save-as various raster formats like .png, .jpg, .gif, etc. but those have to be of high resolution for printing because the pixels are "fixed" in raster file formats, whereas vector images can be resized without losing quality.