You can’t pick a random cell from a repeating group (that was just a theoretical example - that a cell can’t be random if it’s the current cell - it’s either one or the other).
But you can select the data source to be a random item from the repeating group’s list.
It’s not clear what you’re trying to achieve here (perhaps a link to the page would help clarify).
It seems you’re trying to get a random user’s cover image to display on a specific user’s profile page?
In that case you’ll use the button within the repeating group to send users to the ‘dynamic_profile’ page and send the ‘user’ of the current cell.
If you want to display a random user’s cover photo on that page you just need to set the data source for the image to be a random user.
There’s no need to even involve a repeating group list here - just do a search for users, and choose a random item.