May be a bug in this... Rated 3 out of 5 stars
Works great, but only for the originally opened Seamonkey window and the window from which I installed it. Other windows show images centered on the ugly background, even if they are opened from the 1st window after this addon is installed.
I have not yet tried it after closing & restarting Seamonkey. With my workflow that may be a couple months from now.
Tried it on another computer and it seemed to work fine. (Still haven't restarted browser on this one.)
I managed to reproduce this, once, but I have no idea how I did it and couldn't do it again. A restart (or disabling and re-enabling the extension) may indeed help.
Do you have the DOM Inspector installed, and know how to use it? Could you verify that the windowtype attribute on the window is set to "navigator:browser"? (DOMi > File > Inspect Chrome Document > select a broken window > select the #main-window node > select "View > Object Viewer > DOM Node".) I can't see why it would be different, but it's _possible_ Seamonkey does something strange with the windowtype, and if it was different the window would get ignored.
There is a new version in the works that uses a different method of detecting images. That might work better once I get around to releasing it.