In P4 it is better to load a Flat/One sided square and apply the photo to that as a material. To match the Photo size, increase the Square's X/Y scale. eg. 600W x 800H = X 600% x Y 800%. Then increase the Main scale if you need to. This won't pixelate as bad as applying the photo to the background. Now if it is a small photo, your going to have this problem no matter what, just not high rez enough.