Out of curiosity, you plan on putting the photos on a media card right? And not the device memory... If you start loading a bunch of photos onto the device memory, you are going to run into problems.
if you only intend on viewing them on the bb itself, there is no reason to go larger than the native resolution of the screen.
If the pictures are jpg's and small enough in size, I'm pretty sure the BB will scale them down anyway, so if you're not pressed for space, I wouldn't worry about converting them at all. At that rate, if you wanted to actually give / make copies / need an emergency backup, you can just pull them back off the card.
