why not 1/file? After all, a filing system is a database.

(or is it the space overhead of the fat filing system?)

On a linux fs I'd use a hash/heirarchy approach.
Take the unique info (x,y,z), hash it to a unique fixed length (think md5 hash) then create a directory structure to have no more than a few thousand files/directory. FAT32 may break with fewer files/dir so consider that.

eg (12,992,10034) -> AE4 FF3 25F A1D
create \db\AE4\FF3\25F\A1D.bmp

Alternatively use a dbm (gdbm)
http://en.wikipedia.org/wiki/Dbm