I have joined this forum specially, purely because after my recent purchase of an AVH-P3200BT, I couldn't find anywhere via google an answer to the question of perfect background images for my unit. I assume this should also be applicable to the AVH-3200BT, AVH-P3200DVD, AVH-P3100DVD, AVH-P4100DVD and the AVH-P4200DVD. Who knows, it may also be applicable to the whole AVH series..? Here is my experience...
The native resolution is 16:9 ratio, just like normal widescreen, so lets take a 16:9 resolution, for example 1440x810.
This resolution does not work as a background, it does not fill the screen and appears squished. I found that the Pioneer unit was taking photos at 16:13 ratio and squishing them to 16:9. Therefore, what you have to do is take the original widescreen picture of 1440x810, and using a photo editing program, stretch it to 1440x1170.
This is the closest I have ever got to filling the screen with a normal looking ratio.
Convert the following picture from this:

To this:

And this is how it looks on my pioneer (after I cropped to my preference):

As you can see, the pioneer has taken the stretched 16:13 ratio and squished it to 16:9. This is the closest I have ever got to filling the screen - It is not exactly pixel perfect but looks damn good.
Which leads me on the next issue, blurry pictures, poor quality or even flickering. I found that the smaller the resolution, the better the quality got to a certain extent. As long as you always stick by the 16:9 to 16:13 rule you should be good. For example, stretch 720x405 to 720x585 and it looks better on the pioneer screen than the 1440x1170 does. I actually use as low as 360x293 for the Jeep background you see on my pioneer - this created the best quality picture. I also had to darken the picture as it was too bright once the pioneer had used it as a background.
So, in summary, for best full screen background results:
(1) Edit your favourite picture to a 16:9 ratio (example 720x405)
(2) Now stretch it to a 16:13 ratio (example 720x585)
(3) Place on your usb/sd card etc, view on Pioneer and Capture for background.
By my calculations, the following 16:13 resolutions work well, but I use 360x293 for best results. 360x293/450x366/540x439/630x512/720x585/810x658/900x731/990x804/1080x878/1170x951
/1260x1024/1350x1097/1440x1170.