I'm not a professional photographer (or a lawyer), but have read a number of discussions on copyright of photographs.
The consensus seems to be that if you put photos on the net, then if someone wants to 'steal' them they will.
With regards to copyright - unless you explicitly give copyright to someone else it belongs to you as the photographer (exception is if you are taking photographs as part of a regular job, when it will usually belong to your employer).
Can someone take a copy of your image, edit it and claim it as their own without your permission? - no (it may be OK to take a small portion and include it in a larger work, I'm not sure on that).
If they do, can you do anything about it? - In theory, yes.
You can demand they stop using it, pay a licence, etc. but they may just ignore you.
You can take it up with the web hosting company (but don't expect much luck if they are based abroad).
In general, try to avoid posting images at high resolutions - if you post at 800x600, for example things look fine on the web, but won't have the detail for larger prints, etc, so are less likely to be used commercially by someone else.