It's important that you consider the size of the pictures that you use.
For example, it'd be ridiculous to use a 2000x2000 picture for the panel background. The result wouldn't be better. You'd just use a lot of memory, and the program would probably slow down.
It's even worse for the grid cells. The grid cells are rather small, so if you use a much bigger picture, that picture will be scaled down and become hardly recognizable.
So think twice before adding a picture, and don't hesitate to scale down or crop