If the pictures you are inserting are all in the same format (3x4 or 16x9) as you are presenting in, and in the same orientation (landscape), the ideal resolution would be the same as your Main Display, very likely 1280x720 (recommended 16x9) or 1024x768 (recommended 4x3). If you are entering one image per cue, you can avoid having to stretch to fit by applying the image as a background rather than an object.