There are a couple of different programs that will do this, but I have not found a free program. The one that I used is Wondershare Dr.Fone for iOS. I like it best because it's the only one I've found that will work without the iOS device hooked up to iTunes. I know that it will deal with an encrypted backup for a device you no longer have because I found myself in exactly that position a couple of months ago. The combination of an encrypted file and not having the phone may mean that you're stuck unless you buy a package. I decided I really wanted the pictures and ponied up.
It probably goes without saying, but just in case, I have absolutely no connection with the company at all...