First of all, make sure your image is 72 dpi (dots per inch) this is a setting in Photoshop under image size.
An LCD monitor of any type can only output 72 DPI, any "extra" pixels are discarded by the monitor and the
resulting image is anyone's guess. I had a similar problem with my F series AVIC system even after resizing,
so what I did was to put the image on a black background. That way, even when it didn't fill the screen, it looked
good.
I used 72 dpi & 800 x 480 pixel size on my AVIC F940BT this morning and it came out really nicely
I haven't attempted to load a custom splash screen yet, but I am well acquainted with digital graphics...
72 dpi setting has nothing to do how an image is displayed on a screen. It is either displayed at the screen's native resolution, pixel for pixel, or it is scaled, i.e. pixels are interpolated to whatever number of pixels it appears on the screen. How the latter is accomplished is dependent on the rendering engine (can be either hardware or software, or both).
Another issue is the screen's pixel density is not the same both vertically and horizontally. For example, I have loaded a custom splash screen on a Kenwood DNN990HD... and while it has an 800×480 display, the vertical and horizontal pixel densities are different. When an 800×480 image is loaded, it gets stretched horizontally...
I have a 8000NEX which has an effective display area of 156.6mm × 81.6mm (exactly the same as the Kenwood DNN990HD). Divide those dimensions by 800 and 480 respectively, then divide the former result by the latter and you get 1.15 and change. So to get the display to show an image that is not stretched horizontally, the image has to be created at 921×480 pixels, then shrunk horizontally to 800×480 pixels.