The new photoset sizes.
Because of the increased width as well as the smaller padding, when you try to place 3 images next to each other they would have to be 177.3 pixels wide each, which is impossible hence the 1 pixel gap on the right hand side.
i’m reblogging myself to point out that if the padding was 3 pixels or 6 pixels instead of 4 three images would fit at 178 or 176 without weird extra pixel gap