I found a fix, it will be in the next version. Should be a day or two before it comes out.
From now on it will send the URL of the image you've selected in the list of image sizes. Thanks for bringing it to my attention. If you want to patch your version of OT until then, just replace option-tree/assets/js/ot-admin.js with the contents of this gist file https://gist.github.com/31e1e6d7f860aa463616