Have you ever broken the copyright rules?

What copyright considerations should I keep in mind when working on a personal blog?

Typically, if you want to be 100% sure your work is not stolen, usually don't post it on the internet. As John and DA01 pointed out about watermarks, stamps, etc., can be cut, replaced, or somehow excluded from the image file. If someone really just wants to use your image without your consent, they will and chances are you will never find out. A lot of people use images (in presentations, to show around, whatever) found somewhere without referencing them, which is strictly interpreted and is against copyright rules. However, there is the concept of fair use, which enables resources to be used for educational purposes or the like.

All of these points aside, there are many rewards for actually showing off your content on the internet: for the few people who can use your work (and who will actually try to commercialize it?), Dozens will see it, some will Leave a comment and there will even be that one person who will be inspired by your work, create their own, and give you credit. If you want to go in that direction, consider using a Creative Commons or Copyleft license (rather than strict copyright) that fully protects your work so that users can share or customize it.