However, none of these services provides a encrypted connection, which makes me nervous about the safety of my personal data and images. Should I be worried ?

The connection is of little practical importance, IMO. The odds that someone is stealing your images by intercepting your communications is pretty low, given there are so many easier ways to steal images. You and your images would need to be of spectacular importance to warrant the effort required to tap into your connections.

What's probably of more concern (to you) is the potential security of the servers that are holding the images while they are being queued for printing. Again, unless your images are of considerable value and you have reason to worry about their security, it seems unlikely that you can do much about it short of actually printing them yourself (and as a pro that's an option to give consideration). Printing yourself would effectively bypass all your concerns about internet security.

But keep this in perspective and if your images are valuable or potentially valuable you probably would be better to register them for copyright (i.e. formally). This would at least give you some hope of financial recovery if your images were stolen and used by someone else without your consent.