I would strongly recommend using SVGs for icons. While UNA will handle @2 resolutions, using bitmaps for icons complicates everything. For example, one icon source file may be used in certain menus display modes at size of, say, 1rem (or about 16px), while in others it may be stretched to 3rem or more (about 50px). SVGs keep you covered and would be much more future-proof than bitmaps. Note, UNA.io is using SVGs for the menus.