Swapping pictures did nothing, and the keyframes were not why
A track built, two keyframes placed, both of them holding the same photograph. From the outside that is swapping pictures does nothing. The keyframe machinery was never the missing piece, and neither was the animation.
The picker that was never there
Uploading adds a layer. Every card in the elements panel adds a layer. The properties panel offered nothing at all — so the field that holds a picture could be given a line on the timeline, and keyframes on that line, and never a different value to travel to.
12 bundled pictures on 4 shelves — photos, backdrops, faces, boards — in the properties panel of any image layer, with the one the layer is showing marked.
Folded shut, the way the panel below it is, and for the same reason: the properties panel is long already and changing the picture is not what somebody comes to it for most of the time. The closed row carries the one fact worth having, which is the picture itself and its name.
A slideshow is a pick, a move and a pick
Picking writes through the ordinary patch every other control writes through, so it inherits what every other control has. With auto-key on, a pick is a keyframe at the playhead. Five photographs in one frame is: pick, move the playhead, pick, and three more times.
A picture steps at its key rather than travelling to the next one, which is the only honest thing a photograph can do — there is no halfway between two of them. To cross-fade instead, put a second image layer over the first and key the opacity, which does travel.
Measured at five, with five different photographs on the canvas at five instants.
Uploading, where picking is
Putting your own picture on the layer already existed. It was an unlabelled icon among ten in the floating toolbar that appears over a selected object, reachable by hovering it and by nothing else, and with no accessible name at all — unlike the buttons either side of it.
The capability was never missing. The place to ask for it was. It is offered in the picker now, where somebody actually looks for it, and the button on the canvas has its name.
Only pictures the deployment is carrying
The list names only files the repository actually holds. An untracked picture sits on one laptop and in no deployment, so offering it would be a card that works in development and nowhere else.
Two tests hold the list and the folder together in both directions: a picture offered and not carried fails, and a picture carried and not offered fails too. The second one matters more, because it is the one nobody would ever notice.