If you have a pictures folder and videos folder on your sd card, the gallery should be able to see them. It sounds like a hierarchy issue to me, but I could be wrong. You can download Just Pictures from the market and see if it can view your pictures. I use that to manage my pictures. Astro can usually open pictures too. If neither of these can open the pictures, then I would say the pics are corrupted and would transfer them from your pc to your phone again. If you do that, I would suggest clearing the data in the gallery before reloading the pictures.
This issue on your phone should be pretty easy to resolve since I'm pretty sure it's an issue with where you have the pictures folder in your sd card or, the file names are messed up somehow.