Hi,
Okay... here is what the problem is....every time you add more pictures you are adding a new folder...the root area of the drive is only so big... and the folders have used up all of that area...therefore you cannot add anymore folders and thus no more pictures...
Here is what you need to do....
Download all of the pictures back to your PC and place them all in one folder...then completely reformat or delete all of the folders on the photo frame...this will delete all of the pictures also, but you have them saved on the PC, so don't worry... it would even be a good idea to save them on an external hard drive also...
Once you have everything reformatted and cleaned up, then reinstall the pictures to the frame and you will be okay to add more again...
Here is a article that will also help you with this...
heatman101