When I change the picture size from thumbnail, to medium, to large; it doesn’t change to the correct size. Thumbnail shows up bigger than medium. Thumbnails are way bigger than 150 x 150, Medium is also way bigger than 300 x 300
I believe this may be bug and have filed a bug report. In the meantime, you could try adding a custom class to the Post module Settings > Advanced > Class. Then use the following CSS (remember to replace my-class with your own class).