I had this same issue with my AVIC-x920bt. I was looking in all the wrong places for the answer.
The owners manual (Pioneer site, or any of the online retailers sites) list the information you need in the Specifications section.
In the case of x920bt, the screen size and resolution are 800x480 @ 72dpi. That isn't a "standard" size, so almost any pic you download will require re-cropping/resizing.