The image display behavior you described is usually a result of heavy image size. By heavy, I mean 1mb or greater.
What you can do to avoid or at least minimize the effect is to optimize your images. Please read Image Compression section in our Performance Guide.
You might also want to use Progressive JPEGs.
Aside from optimizing your images, we can do nothing on our part as our theme does not have image optimization features. It is a feature provided by Image Optimization plugins.
Hope that helps.