When I was sharing lots of photos online I would use photoblink.com. I haven't been there in a long time, but it was a fun site for general photography, maybe not so much documentary photography.
The only real way to prevent people from stealing your photos is to not post them, since a screen grab gets around anything but an annoying logo over the top of the image. You might want to post only low resolution images, but people will steal those too if they want the image badly enough, and then you don't get the satisfaction of exhibiting the photos in high res.
Photoshop does watermarking, IIRC, but I've never felt a need to watermark my work. In fact, cross you fingers that someone does use them without permission and then just bill them a usage fee, hehe.
Frankly, I don't see you getting paid for any of those shots without people in them, and this late after the event. To sell images from a disaster you must include people in your photos. Human drama sells much better than shots of random wreckage. You also have to get them to market about 60 seconds after the event.
When shooting news it's every shooters dream to capture the event as it is happening with all elements of the event in the photo, i.e., people running from the tornado with the twister and a path of destruction in the background. If you can't get than then you shoot people involved in the immediate aftermath and rescue efforts, again, with as many elements of the story in the image as possible and concentrating on the human drama.
Images that tell the story of disaster and human suffering (sadly) sell best. Try to avoid generic shots that don't tell the complete story or could be from any event. Bent wreckage or a demolished house is about the last to sell, but if a tornado victim is rummaging through the shambles of their destroyed home then the images marketability goes way up.
Kinda cold and mercenary, isn't it?
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