Does it happen every time, most of the time, or randomly?
What format are your pictures in (JPG, PNG, etc)?
How many pictures are in the folder?
What sizes are the pictures? (eg: 2MB – 6MB)
Can you reproduce this on a fresh load of WiiMC, and directly going to the photo tab? Or do you have a video or music loaded before?
Can you try removing all but one picture and see if you can reproduce the crash? If you can’t, could you try adding 5 more and see if you can? If not, 20 more and see if you can?