- Re-size and crop the images in such a way that all of them have roughly the same scale, and that the eye line more or less lines up
- Choose active pictures: people pointing, making a gesture
- Have people look towards the center of the page
- Harmonize the background (light or dark)
- Harmonize the color, make them black and white if necessary
I stuck to most of these suggestions (not all) in the example slide below: