You would need to modify your code for such things in the showphoto.php file to achieve that. Good code modification question
Basically you need to make a new varialbe for the thumbnail because the existing function uses $imgurl in showphoto which shows the photo you are viewing.
Whereever you see $imgurl defined try making a new line like so
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You can then in the ubbcode etc define lines replace $imgurl with $imgthumb