There are two groups of people. Those who love Ubuntu and those who hate Ubuntu. I am one of the latter. The problem is, Ubuntu as a whole brings nothing to the table as far as distros go. It's built on Debian, uses a custom (and ugly) theme and that's about it. It does nothing new... yet it's immensely popular. The reason people are annoyed is because while people continue to flock to Ubuntu, argueably better distros are being ignored.
I have used Ubuntu and I was impressed by how much of a pain in the ass it was to actually use. For newbies, openSUSE is far easier to deal with. One benefit that Ubuntu has is the large community, and clear documentation. Ubuntu is just another fad, really. Something new comes out, people flock and become infatuated with it... it happens all the time.
I think what drives the author of that wallpaper and other Linux users mad is the fact that MANY Ubuntu users act as though it's the only distro on the planet. Constantly you will find news stories, "How to do *fill in blank* on Ubuntu", when in fact it could be done on any distro. Articles like these come out every single day, and it just goes to show how Ubuntu users are still complete Linux noobs.
That's about it really.