What's the max amount of files per directory? (Linux)

I have to store a lot of image files, but I'm not sure about how to organize my directory structure.

Is there a limit of files per directory, or directories per directory?

Also, should I create the filenames ra开发者_如何学Gondomly so they are not guessable? (so I don't get my bandwith hammered by someone trying to download all the files)

Wikipedia has a nice overview: http://en.wikipedia.org/wiki/Comparison_of_file_systems