How to combine multiple images features
Your initial intuition seems pretty good to me. A pretrained image classifier like VGG19 or ResNet, as included in the Keras library will yield simple vectors if the include_top parameter is set to False upon initialization. You can then use a method like PCA to reduce the dimensionality to 255, or whatever number your algorithm can handle. Averaging the vectors before or after this step will allow you to merge the photos. You will need to test which works best for your application.