If anyone else stumbles upon the same issue, here's what I did to fix it.
I made a new imageView in IB, removed the old one, and only changed the contentMode-value and clipsToBounds-value. In the tableViewCell.m init-code, I set the placeholder image, and voila – everything works like a charm.
Although I don't really know what caused it, it may have been the Clears Graphics Content checkmark's fault for being set (oops).