If compiled with USE="-gtk qt4", neither /usr/share/icons/*/*/apps/transmission.png(.svg) nor /usr/share/pixmaps/transmission.png icons are installed, although one of these is needed by the transmission.desktop file
Yeah, that's because the Qt4's desktop file is provided by Gentoo..., I've fixed this without revbump in 1.75 so that it manually installs a icon too