Try using this addon https://addons.mozilla.org/firefox/addon/transparent-standalone-imag/

still not working :(

It seems I found a workaround. You need to install Greasemonkey, go to this link: https://userscripts.org/scripts/show/153158 (Standalone Image Background and Transparency) and install script. After that you can change background options of standalone images. I always thought that dirty gray background, instead of transparency, was something like a bug, but it is just retarded feature =/