When saving a theme the individually set toolbar images are not redrawn when the theme is reloaded, only the Standard toolbar image, when set on a toolbar, appears. The actual images are still there to be used though.
Standard toolbar image is there after a reload
But a custom toolbar image is not, although its still in the list as all images have been saved
Themes don't save any connection between individual toolbars and background images.
(There would not be much point since no one else you're sharing the theme with is going to have a "Spacer Bar" toolbar, for example.)
spot the empty toolbar
seriously though, perhaps an option to save/restore individual toolbar images would be nice.