Hi Emmanuelle,
That’s really weird. Facebook is the one that dishes up the images and we are sometimes at their mercy. That’s why it usually is a square image that comes over. But yours is getting chopped up beyond that. And it kind of looks to be a part of the map that was on the Facebook page, maybe? Perhaps because there really isn’t an image on the page something isn’t working right? Are all the events you are having trouble with ones that don’t have images? That would be an important bit of information.
In the meantime, have you tried deleting the event and trying to re-import it?
— Julie