I found the issue within the code of this plugin, which is basically that if you have an image that is (for example) taller than it is wide and it is followed by an image that is wider than it is tall, it is still using the calculated resized width of the first image on the second one instead of using the maximum width that the newly recalculated height is based on. In other words, it doesn’t reset/set the variable when it should.
Another way to fix it would be to make sure that your uploaded images all fit within the dimensions that you specify, and then it would never try to resize them at all.