When the newest version came out, I set up DisplayFusion to swap my wallpaper images out every 10 minutes (on each monitor) randomly from my Flickr account. Today I got notice that my hard drive was full. After a bit of searching, I found 40GB of images in the DisplayFusion folder.

Is there a way to limit this? I understand I'm likely changing my images more than most people, but is it really necessary for it to keep every image it displays?

Easiest temp fix is to make a .bat file that points to the folder and says Del *.* inside of it, put it on your desktop and run periodically. I am guessing Jon overlooked it, I can almost guarantee that he will fix it in the next version. Knowing him, he will....
As for having your WP change every ten minutes.... Mine changes every 2 . All from a folder though, so I had not run into that issue..

BTW to make a .bat file,

Create a new text document,
Write DEL then the path to that OMG HEUG folder, and then put *.*

So if it was in C:\DF\flicker the bat would read:
Del C:\DF\flicker\*.*
You can additionally add /Q to simplify the process, but please only do so after the first run, as it is quiet mode, which you have to be careful with on a global wildcard. If there are subdirectories in the folder that you also want to get rid of, also add /S
Finally, save the file. Rename it so that it ends in .bat if you have file extensions on. You must have them on to do this. The icon will change, and that is how you konw that it worked. Run the file, and it will empty the folder. Do not be alarmed if you see a DOS window, that is what .bat do.