Don't listen to these naysayers. There are definitely opportunities to volunteer as a photographer abroad, you just have to use networking or major google-fu to look out there. If you look up international opportunities, there are a plethora of websites.

http://www.transitionsabroad.com/publications/magazine/0309/volunteering_for_an_ngo.shtml <-- Here is an account of a photographer who just straight up e-mailed an NGO and asked them if he could volunteer as a photographer. I suggest you read that and do the same.

Maybe you should check out this one -->http://thegivinglens.com/ which is a Photography NGO that is probably right up your alley.

Here's a photography volunteer opportunity in the Philippines https://www.kayavolunteer.com/project/media-photography-project-support-ngo-philippines/

Here's a company that has storytellers do fundraising http://www.globalpopulace.com/landing-1

Now finding a specific teaching opportunity for photography might be hard to find, because most places who want volunteer teachers are developing countries, and their students won't be able to afford photography equipment. But you could be an art teacher- here are a few listings http://www.gooverseas.com/volunteer-abroad/arts

There are not very many programs that will pay for your airfare without giving up a long term commitment, many of those are paid teaching jobs. Some volunteer jobs do pay for airfare but you have to fundraise a lot of money usually.

If you want to do documentary work, you could also find networking to get you a spot, I have a few friends who have done documentary work while living abroad working with vulnerable populations but it was mainly through networking and not a specific job listing. These people had already lived abroad for a time or taught English so they knew people in-country already.

I promise you there is an opportunity out there to fulfill what you're looking for. Maybe not completely but if you look hard you'll find something.