Since you have the thumbnail, you were in the advanced posting area. I am not sure I know the best way yet, but how I did it was:
I wrote my required 10 letters of message after I uploaded my pictures. I then previewed the post. clicking on the thumbnail picture, i opened the full size picture, this might require clicking on the popup picture and getting a new tab opened. Once I had the full size opened, I copied the url location of the picture - the "http://www.justusboys.com/....." - with the address copied, I went back to my posting and clicked inside the post and then clicked the image icon - the small mountain - this loaded an image command with the cursor blinking between the two commands. I copy the address of the full size picture between in that space. I always do the preview a second time to insure it is correct. It is a lot of work, but hopefully people will enjoy the post more.