You can change the portrait associated with any person in Family Tree simply by clicking on the person's current portrait (it may be the default placeholder image if no portrait has been set). Then you'll see options for selecting from currently attached photos, uploading a new photo, or choosing from your gallery.

Unfortunately, when someone changes the person's portrait, there is no entry made in the change log. So you have no idea who set the current portrait. If the current portrait happens to match a photo memory attached to the person, then you could look at the memories attached to the person and see who contributed that particular photo. But even that doesn't prove anything, since one user can attach the memory, and a second user can select that memory as the portrait. So there's really no way to know who to notify of any problems with the portrait -- just do your best to fix the portrait.