OK, I've found a way of doing this. It's not ideal but it works. I'll use the Triumph thread as an example.

Right click the picture in that thread and select Properties and copy the URL:

https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showimage/id/1243/type/m/Bonnie2_jpg

Now change the /m/ to /f/ in the URL.

https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showimage/id/1243/type/f/Bonnie2_jpg

The code in the URL seems to be:

/t/ = thumbnail
/m/ = medium
/f/ = full