The ebuild passes the right configure option for use=+png for the non gtk version, namely $(use_enable png screenshot). For the case of the gtk gui it passes $(use_with png) but this should be $(use_with png screenshot). The consequence is that the screenshot function doesn't work in the gtk gui right now.
Created attachment 247150 [details, diff] Patch for games-emulation/snes9x-1.52-r1.ebuild
good catch. fixed in portage.