Importing posts — picture size issue

Hello, I recently moved from WordPress.com to WordPress.org. I imported my old posts into the new blog fine, but the pictures in the posts aren’t the right size — they are the original size instead of the size I chose for the post (usually medium, sometimes thumbnail size). I’ve tried to manually edit them back to the size, but to no avail: even when I change the size of the pictures in the editor, the actual pictures on the post remains the same size.

What is the problem here? How do I get the pictures back to the size they were on my WordPress.com blog?

Thanks in advance.