If I recall correctly, you have to go into the Media App - Menu - Explore - Mediacard (assuming that's where you store pictures) - click on the picture you want hidden - BB key - Properties - Check Hidden.
I had the same problem and this was my workaround. I will go check now and if I am wrong I will edit my post.
Goto the media app, click bb button, goto options. The top options are for pictures, click exclude folders, goto the folder you want hidden, highlight it and click bb button then exclude. you will not see it anywhere, to reshow do the same steps and click include.