I would --- myself --- explain to the client that his way is causing trouble with the lightbox. That if he insists to have the size in the name instead of the body then to use in. for inches or ft. for feet

otherwise try:
includes/templates/your-template/templates/tpl_modules_main_product_image.php


there was I was sure another file that actually said what you are seeing but can't seem to find it in any of the fiels that are installed by lightbox.

But there are many software programs that any type of punctuation in the name will cause all sorts of strange reactions and the best thing is to get your client to realize that the size is not necessary in the name....or needs to be spelled out

these are qhat a lot of software programs will not allow in the name just like naming a file for zipping it up...no spaces nor those characters are best:
$/\\:*?"'<>|` &