After messing with this for a while, I think that the number of images that can be accessed is related to the size of the filenames. Smaller names seem to mean more images can be stored. 308 is the most that I can get on it- this was using one and two character filenames (plus extension).
My Juicebox is all green, no LED, 2.6, checksum: 26584d02, 8MB RAM, mp3: 1.0.5, firmware: 2.0.8
I wrote a small utility in CSharp to help me with the renaming. It can rename and clone files. Source is included. To rename, give it a directory and click the rename button. Note: All files in the directory will be renamed to short names! To copy a file, give it a source file and click the copy button. It creates a bunch of copies of that file, all with short names. The copies are placed in the application's directory.
Here's the utility: