Can I downsize those images at this point or is it too late? The modify option is long gone on those posts.
Yes, you should be able to make the photos smaller, and reduce the quality.
Remember, you've just "linked" to images you have stored on your photo sharing site (they aren't stored on this forum), so as long as you go to that site, and edit those photos, and don't change their names, the newly edited photos will be updated in this forum.