I'd like to set a static, custom wallpaper in the Photos app but I want to make sure the entire image will be viewable. What's the proper height and width, in pixels, for such an image?
In this section, you will apply an image to the Panorama control. For this topic, a sample image of samplePhoto.jpg is used. Use a properly sized image in your app. Background images should be between 480 x 800 pixels and 1024 x 800 pixels (width x height) to ensure good performance, minimal load time, and no scaling. If your image does not have a height of 800 pixels, it will be stretched to that height without preserving the aspect ratio.
According to the page, it applies to WP8 and WP7.1; however, I just created a new Panorama Project in Visual Studio, and checked the properties of the BackgroundImage.png in the default template and it is 1536x1280.
I recommend you use these numbers to start and then answer your own question here when you find out the exact measurements.