I’ve searched extensively, but the feature seems to be missing.
Currently, you can drag-n-drop several images on the timeline, and even select and move them around no problem. However, when you try to stretch or shrink them on the timeline, only the last picture in the sequence length changes.
The idea is to be able to select multiple pictures on the timeline, and evenly resize all of them at once to fit a certain length. For example so the slideshows starts and ends with a piece of music on the other track.