I do photo manipulation too but only when I've transferred the JPEGs to the PC -- I couldn't be bothered faffing about with images on a dinky little handheld screen. But if you do want to do it in situ
on a BB, your options are severely limited. You could maybe check out Radical Draw at BB Appworld, but I think its capabilities are only realised if you have a Storm i.e. a device with a touchscreen.
EDIT: Actually, maybe PhotoColor
would be more useful to you.