Glad to hear you're limiting the number of files (see: Randomness of Flickr Random Images) but could I make an alternative suggestion. Tonight I noticed that one of my backup routines was taking sometime to complete, it normally runs quickly so I checked it out. It turns out that I've got 1391 images stored within the Display Fusion folder (that's not a problem), my issue is that they're all 18MB or so PNG files, totallying a whopping 18.8GB!
Looking through them I noticed most were duplicate images so my recomendation to help with the randomness issue and bloated FSO storage is to name the files by the Flickr ID, if it's already there, don't save it again, otherwise this is potentially an bottomless pit. I'd also store them as a reasonable size (I uplaod at full size to Flickr -3264 x 244 but have a screen res of 1280 x 1024. Ok I have three so it could span them but resizing the image on download would dramatically improve the folder's size...
Sorry about this, it was an oversight when I added the random image feature. The next version will automatically clean out the unused images so you don't end up with a giant folder like that.