As a rule of thumb when authoring, bigger images work best for scene and scape images. The user experience tends to be best when they are rectangular – something like 960 pixels in width by 640 pixels high. It doesn’t have to be exact.
Because the display is big enough, photographic images taken with DSLR cameras can look very nice as scape and scene images.
Rectangular whitepoint images can also work, but because they have more limited constraints for display, square images will usually work best. Tall rectangular images can also work as whitepoint images. You can limit the width of whitepoint images to about 500 pixels and get great results. At 500 pixels wide, shoot for at least 500 pixels in height.
And, because of the smaller display area, camera phone shots work just fine as whitepoint images. They can also work in scape and scene images, but you may not get the effect desired. It really depends on your subject. The good thing is that Whitepoint authoring makes it easy to switch out and update images at any time now or later.
Finally, for all images, 72dpi will likely suffice – high resolution is not only unnecessary, it may even produce adverse effects in your display. Test and take a look at your subject on different displays. Don’t be afraid to test an image and switch it out or update it later.