My suggestion will be different from others' ...
I recommend that you use CarbonCopyCloner to do both your initial backup, and your incremental backups later on.
CCC is "customizable" -- when you set it up to do a clone, you can "de-select" folders and files you DO NOT WANT to be copied. So, by a process of exclusion, you can deselect EVERYTHING EXCEPT your iPhoto library.
CCC will then back up the library (and ONLY the library) to your target drive, and during incremental backups, it will backup up ONLY the "changed files" within the iPhoto library.
In CCC, I believe you can set this up by creating a "filter" for your drive that includes ONLY your iPhoto library. The initial setup may require a lot of "un-checking", I've never tried this myself. But it looks to be not that difficult to accomplish.
BTW, I don't think you need the latest version of CCC with which to do this.
I believe you could download the older "free version" of CCC, "click-through" the disclaimer warnings that it hasn't been tested with the latest Mac OS's, and do just fine.
The version of CCC you want to try is 3.4.7 and you can download and use it for free from this address:
http://www.bombich.com/download.html