I’d say you basically have two options.
1) See that option in that screenshot that says “Thumbnail Alignment” — You’d change that to centered.
2) But the main problem I think is more that the thumbnails are too large for the narrow area you’re trying to fit them in, meaning that two can not fit on one line. So, you can play with that “Thumbnail Size” option to adjust that. “Small” is the cutesy name for the largest thumbnail size in the mini post grid/lists — Small, Smaller, and Smallest.