If you put your image on the net it is likely to be copied. Not much you can do about that. Most countries have a fair use policy. Generally, only companies using copyright images (illegally) for commercial reasons will be sued.
they can still edit it, but it'll be awkward to see multiple watermarks on the image. so they will normally leave it as it is. take a look a what shutterstock do to their images (you can search it if you have time). they put their watermark on the images so people would have to buy it to use it properly.
I agree to Jay. Putting watermark is the only thing we could do without assurance though of not being stolen, but at least.
In my photos, I usually edit it in picnik.com (which is actually closing their own site and moves to google+ come April 19). It is free online editing tool which functions similar to that of photoshop and other.
Just to give you an example, try viewing my photos on two blog sites. Just to give you an idea. And visit picnik.com while they are still open and free.