Netiquette

I don't know about all of you, but I've come across several blogs or online galleries that have taken my work without permission. Somehow, my work even got used on the "About.com" Clip Art page. "About.com" is supposed to have guides who are recognized experts in their field and can provide trusted information after thorough research. Last time I checked, illustrated blogs were not a clip art image resource. Since it has happened a lot in the last month, I realize I am going to have to add a watermark to all my images from here on out. What do you guys think? Have you ever found your work on other blogs or websites that didn't ask permission or didn't give you credit?