Please use SVG, iconfonts or spritemaps for the adminarea
Can you please provide some arguments why would be this necessary?
PNG is fine because all icons are static-width, not responsive, so quality is not lost by scaling.
And spritemaps are just a performance improvement so that less HTTP requests are done to download the images, but being an admin panel (users do not have access to it) the number of HTTP requests is not that important as everything will be cached anyway. Also, spritemaps are harder to edit and maintain.