How large are the images? If they are any larger than what the screen size is, you’re wasting memory and is probably causing the inability to add more. Just as there is a finite number of songs you can put put the device in the form of a database limitation, it logically follows that there is a finite amount of space allocated to pictures/images. In fact, it could be that these are catalogued in the same database as the music info, and so you have reached the limit.
I would try reducing the size of a few of these images and see if it frees up any space to where you can add a few more. If it works, then you have solved your issue.
