I can't seem to figure out why this dropdown is only showing "Full Size" as the option. I have the default "thumbnail", "medium", and "large" set in Settings, and I've also added a few custom sizes in my functions.php (using this method):
add_image_size( 'single-post-medium', 515 ); add_image_size( 'single-post-small', 250 );
Here's a screenshot of the area I'm speaking of:
It's in the pop up that appears when you click "Add Media" when editing a post.
I'm on Wordpress 4.1.5.
My issue seemed to be that my thumbnails were not getting created. Images were uploading fine, but no other sizes were getting created. This turned out to be a local environment issue. First thing I did was set uploads folder to 777 recursively. That didn't work. Then I made sure GD was installed and active. It is, I see it when I dump phpInfo. Locally, my thumbnails are still not getting created. Not sure what the issue is there. But as soon as I moved my site to my dev server, the issue was resolved because the thumbnails were getting created.