There may be an easier way to do it, but you can preview your post (before it actually posts) after you've inserted the thumbnails; then once you are on the preview, right-click (if you're on a PC) on the thumbnail and open it in a new tab. Opens up the picture with the actual image URL, not the thumbnail URL. Then delete out the thumbnail from the posting window (don't physically delete/remove itgg from the post though,) insert the image URL by clicking on the "Insert Image" button (it's between "Unlink (which is the globe with a red x on it)" and "Insert Video (which looks like a piece of film strip.")
Should insert the normal picture.
Keep in mind, the server automatically resizes all uploaded image files to 800 x 800 pixels.
I'll see if there's an easier way to do it; I guess I never really looked before