Hi, thanks for reporting the issue. Indeed Gtk3 doesn't resize the images enclosed in a GtkMenuItem. It picks up the right size if different size are available though.
So, for an immediate workaround, I recommend you to tweak the Buuf theme. The problem here is that the Buff icon theme only provides 128x128 icons. If you manually the icons PNMixer use to the size you want (might be 16x16 or 24x24 depending on your font size), then put it in the right place and edit the theme file, I guess Gtk3 will then pickup these icons and it will work. For you to know, the icons PNMixer use are: system-run, preferences-desktop, gtk-refresh, help-about, gtk-quit Otherwise, you can just wait for the next PNMixer version, should be around two or three months. I found a way to workaround the problem, it's fixed in the upstream version now. Best Regards, Arnaud
