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In my opinion, Time Machine is a general program to back up 'everything', on an hourly, daily, weekly, and monthly basis, not really intended to back up only specific files or folders. In fact, Time Machine preferences are designed to help you EXCLUDE certain files and folders, rather than specify which ones to INCLUDE.
There are many alternatives. These are two I've used successfully:
Though not specifically designed for backup, iPhoto Library Manager from Fat Cat Software can duplicate, move, and also rebuild your iPhoto library file. It is available for US$29.95 from fatcatsoftware.com
For backup, I use an older free version of a program called Carbon Copy Cloner from Bombich Software that will permit you to back up files, folders, and and even create a bootable 'clone' of an entire hard drive. The current program release is no longer free, and is available for US$39.95 directly from Bombich Software Store. http://www.bombich.com/
Note the downside that kostby told you regarding the exclusion of files using Time Machine. That would mean everything but your photos. Not an efficient option.
I would suggest just drag and drop your photos entire (iPhoto Library if you use it) to the external HDD on a periodic basis. Perhaps not an elegant solution, but indeed simple.