Check the max resolution of you pic frame (800x600), you may have to resize your pics down in res until they display.
Take one pic that will not display and reduce the size 6 or 7 times on a memory card.( ... Make it a litter smaller each time)
Then load and show each one until it is displayed. You have found your target size...
We use Picasa program to manage our pictures and videos. It's a free program and it works great. It should also help to fix the problem you have. Let me know if it helps!