This is the way it was designed. The Photopost template is placed after the Threads categories. I know I could place the random photos up top of Threads categories. To do what you suggest would require a bit more hacking as in removing a part of the Photopost hack and placing it in essence between the category loop to the next one but on multiple category boards this would look bad. I have like 7 categories so you see what I am getting at? If you must want to begin with the Photopost hack then list Threads do this
Find the UBBTPRINT; up the top of the ubbthreads.tmpl file
right after this and before the echo <<<UBBTPRINT place your line
require ("/PATH/TO/ubbthreads/templates/default/ubbt_photopost.tmpl");