Welcome, @Sineis_Arilla. 
I apologize for only seeing your post now -- I have been extremely sick (almost terminally ill 
) for months and have only recently really started getting better. But now, I would like to explain what's going on.
What theme are you using? The Update Notifier program, which displays the notification telling you to reboot, is looking for an icon with the name un-reboot. The un in the name here is supposed to stand for Update Notifier. Your icon theme apparently doesn't contain an icon with that name, however; it's default policy in GTK, the support "library" Update Notifier uses, to look for a"fallback" icon which lacks the part of the name after the hyphen (-) character, so it looks for an icon called un. There is one -- it's a big, 640x480 picture of the United Nation's flag.
I'll patch it for you and try to get it sent "upstream", if you want. It's pretty easy to fix; nevertheless, if it's not super-important to you, I've got better things to do.
(Source: reboot.c « src - ubuntu/+source/update-notifier - [no description])