The cards a visitor sees first, and the picture on each one.
The front screen is what a visitor sees the moment the tour opens, before they have walked anywhere. It is a set of cards, and this screen is where you dress them.
A tour with no pictures here still works: every card falls back to its name on navy, which is what every tour starts out looking like. Adding pictures is the difference between a tour that looks like a tool and one that looks like somewhere worth visiting.
One picture per card, 1600 by 900 or larger, JPG or PNG. Bigger is fine: the Front Desk shrinks and compresses it for you, so a file straight off a camera works without you preparing anything.
Keep the subject in the middle. The same picture is cropped two different ways, a long strip for the banner and a shorter strip across a card, and the middle is the part that survives both. A photograph with the interesting thing at the edge will lose it on one of them.
You can mix them. A tour where two cards carry photographs and the rest carry the name on navy looks considered, not half finished.
Press Choose a picture on a card and pick the file. The screen tells you what it is doing while it works: preparing the picture, then saving it.
You do not need to resize or compress anything first. If the tour has no capture session yet the screen says so plainly, because a picture would have nowhere to be stored.