My wife walked by, saw it, and asked me "what is that ugly thing on your computer?" I was forced to simply shrug in agreement.
Now, I admit, I do not possess the finest of tastes. My tastes are less caviar and champagne and more bacon and beer, but that's fine for me. I know what I like, and I know that this pink spray is not it. I figured there might be others who don't appreciate the choice of artwork here either, so I took a few moments and figured out how to change it.
If you go into the gnome-control-center (accessible from the top right menu, then clicking the "tools" bubble), then click on "Background", it will open a window where you can change Background and Lock Screen. Clicking on Lock Screen brings up an interface where you can choose from a set of art (either Wallpapers, Pictures, or Colors) that has been hardcoded by the GNOME upstream for your viewing pleasure. Pick one of these pictures and the Pink Panther nightmare is gone.
... but what if you want a custom picture there? I like to customize my desktop. I changed my background to one of the optional F19 backgrounds (the one with the tree frog), and I've been using a custom plymouth plugin for the bootsplash with an animated Hypnotoad for a while now. I found a clever picture of the Futurama splash screen on the internets:
Okay, so maybe that's only funny to me, but I kept thinking that the Pink Volcano reminded me of that. Download a copy of that file (or whatever JPG or PNG you like), and put it into ~/Pictures. Now, it will show up in the gnome-control-center tool when you restart it. Then you can select your custom lock screen.
I can't help but wonder why they do not have a file selector here. Seems like the sort of thing that would be nice. Maybe it is coming soon.
EDIT: Documented using ~/Pictures instead of altering the hardcoded wallpaper XML list.