Hiding a folder is just a matter of selecting it with your trackball and then pressing the bb button and choosing "hide." You can't see it, but it's there. When you want it back, press the bb button and "Show all."
It's not super secure - anyone with a bb knows about hidden folders - but you can't see it just by looking.
Instead of hiding folders, hide the contents. It's easier if you use mass storage mode. Connect the phone to your computer after you've enabled mass storage on your phone. The memory card will come up under "my computer" as a flash drive. Select all the pics that you want to hide, mark them *** "hidden."
To look at them from your phone, open up media>hit menu "explore" go to the folder that the pics are in, hit menu again and select "show hidden"
Thats how i do it too, although, i went to explore, and tried to hide the entire folder that i keep a group of pictures in, but the "Hidden" option was greyed out and would not allow me to select it like i can with individual pictures.