It could quite easily be bespoke. I have nil experience with image upload scripts, i took a look at HTML and there were no giveaway signs of it being a pre made script.
Ye, the owners probably put it together himself, afai have heard upload scripts are pretty easy to code if you have the knowhow, there aren't many cases to consider, just save the files to the server after validation and plonk them in a user folder if needed with a user interface