question here is: what do you want in more details? Just hide from the pictures apps of your phone? Watch it temporarily and hide again? or do you want to make sure NOBODY can watch/read your files.
there are plenty of hiding apps in the market.most of them only change the extension and/or the directory. the solution with the . (dot) at the beginning makes this data only for unrooted phones unreadable. if you are rooted you can access those directories with tools.
Finally I think the safest but of course a little bothering solution is encryption.
If you do it file by file (with any encryption tool like PGP or so) then it is quite bothering to watch them, also be aware that all deleted files can easily be restored as long as they are not overwritten.
I prefer the solution someone above already made: encrypt the whole folder and run it as virtual folder or drive.
I am trying at the moment to get TRUECRYPT, which is a PC windows/linux/mac application, ported to ANDROID. Need some time for this.
greetings from China