Thumbnails after Migration from WordPress.com

Hello! I have recently migrated a large blog from WordPress.com to a self-hosted WordPress installation. However, I’m having a problem with images in posts.

The images on the new install show up just fine, however, the thumbnails are actually the full size images, so this makes the page load very slowly. As far as I can tell, what’s happening on WordPress.com is that the image thumbnail in the post source looks like this:

<a href="https://blogname.files.wordpress.com/2012/01/redneckeggs.jpg">
<img class="alignnone size-full wp-image-4998" title="RedNeckEggs"
src="https://blogname.files.wordpress.com/2012/01/redneckeggs.jpg"
alt="" width="640" height="426" />
</a>

But contrary to what that source suggests, WordPress.com is actually serving up a 640×426 resized version of the image for the inline thumbnail. When I right click on the thumbnail, I see that the image source is actually this:
https://blogname.files.wordpress.com/2012/01/redneckeggs.jpg?w=640&h=426
..which is actually a 640×426 version of the image.

However, at my new installation of www.remarpro.com, the source looks the same (except it points to my new blog domain), but the image source is actually the full-size image, aka:
https://newdomain.com/2012/01/redneckeggs.jpg
..which means that the full size image is being loaded instead of the thumbnail.

However, if I check out the wp-content/uploads/2012/01/ folder, i see that there are multiple versions of redneckeggs.jpg, such as redneckeggs-640x426.jpg. So my question is, how do I fix this? I have hundreds of posts, so there’s no way for me to go through each of them and set up the proper code, and it seems to me that WordPress should be smart enough to fix this anyway. Has anyone run across this and come up with a solution??