I'm a german user of the Dude so please keep in mind that my ability to express myself in english differs from time to time.
I had some problems while importing new images / pictures for new Device-Groups into the Dude 3.1 (fresh system, no imported maps, Windows Vista SP1 x64)
First they were displayed well but when I closed the program and restartet it they disappeared. I mean that the new device groups I created stil existed but there wasn't shown a picture for it. When you entered the properties of such an item there was only the tag "loading..." in the image preview.
Later while testing some things I found out that this only happened to devices which were the only ones in the whole map (single devices) and it did not only happen to importet images. The images and device groups which come with the dude are also showing this behavior. So I think you can test this bug by creating a new network map and put for example only one "Web Server" into this map. The globe image will look fine but when you exit the dude and restart it the globe will disappear.
I also found this workaround: I created dummy devices of the same group and with the same images but only with image size "1" (very small and unnoticable). I made it's background white to make it look like the map-background. So when you got only one Device of it's kind in the whole map, create another one wich isn't "visible" and the bug won't be a problem.
I hope this will help the developers to find the bug or help anyone who had the same problem with the Dude.
INDAVO