There used to be a way to directly set Calendar covers. Today when looking at a picture you can select it as the cover for a folder, album or event; it used to be that there was a fourth choice. But wait . . .
Nice as the Calendar really is, it comes fully to life when you start using Events. Each Event can have a cover, and there is a “cover picker” that lets you pick the picture. You can also do it from the picture side by specifying the Event. That’s the start.
You can also designate an Event as “Event of the Month”, “Event of the Year”, or “Life Event”. In a calendar rectangle you can specify how many events appear; I pick four or five. If you then have say, three "Event of the Year"s those are teh ones that appear in the rectangle and one of those covers will be the one that appear. If you have Life Event that trumps them all.
That already gives you a pretty high degree of non-random control.
There are three ways to create events. The least work way is with Calendar Connectors that pull in “events” from your Outlook, gCal or iCal calendar. So, that ski trip to Tahoe appears automatically and when you tap on it pictures from the associated date range appear. Cool.
If you use folders, as I do, any folder can be “converted into an event” by flipping the “Show in Calendar” toggle. You can then specify in the info panel what filters to apply and whether it’s an Event of the Month, Year, or Life.
Finally you can also create an event directly in the calendar by specifying the date / time range.
Hope this helps.