Hi Tornado and welcome to the forum!
First, make sure the photos are no more than 200 pixels wide, second, make sure the names are in the right format, exactly as they are shown as the Registration in the grid, for example:
SE-DOR.jpg
SE-DOR,2.JPG
Restarting the RB software is all that you need to do - you don't need to do any database changing unless you want to avoid getting a different photo when you click it in the interface.
I don't know if the 200 pixel size is any sort of rule, but I've got 19,000 photos in my folder, so I don't want them any bigger than they need to be!
I can't think of anything else - hope that helps.
Rod