File upload fields in web forms are not allowed to be set by a remote machine (which is what you are asking to do), or http://somenastysite.example.com/ could force sending /etc/passwd or the like without the user even knowing it. The best I can think of suggesting is to create a session of sorts, upload the 13 files via AJAX, JSON, etc to that session, then call the calendar maker on that session.

Unless you can restrict the picture size your customers are uploading (I have jpeg files that print well at 8x10 or higher at under 1MB), or relax your requirement for uploading all of the files from a single page, I cannot see an easy way to avoid some sort of dynamic processing of the files.