[Iñigo]

Originally Posted by Mackx

I am not sure how many files can be in a FAT directory, should the System/_covers/ contains a directory structure?

For speed I'd suggest a structure like:

SD_card/System/.covers
    medium/
        00
        01
        ..
        FF/
            id.png
            other_id.png
            ...
    small/
        [similar to medium]

That's the way some programs work to avoid file system limitations.
If 256 subdirectories are not enough we could use a bigger number.
In order to make file loading as fast as possible we have to think in a map function that equally distributes covers in the 256 subdirectories. I can think of "id mod_of_256" (id % 256). Btw, does % operator exist in C?