By the way, I found out that you can watermark all of your images that you're about to upload to Picasa Web via Picasa Desktop (there's an option to do that before you Sync to Web). I found that feature very useful if you organize your pictures using Picasa (and show them on your blog).
Or some server code that serves a watermarked image if the client IP block is that of Pinterest's caching servers. People see the un-watermarked image on your site, but a watermarked one when browsed on Pinterest.
Problem is, copyright is implied on the part of the creator. It does not need to be applied for, nor is there any requirement for a copyright notice.
Just because you didn't watermark your image doesn't mean the copyright now belongs to Pintrest (or imgur, or Google+, or Facebook, etc) because people who didn't hold the copyright and didn't have the standing to give it away posted it.
By the way, I found out that you can watermark all of your images that you're about to upload to Picasa Web via Picasa Desktop (there's an option to do that before you Sync to Web). I found that feature very useful if you organize your pictures using Picasa (and show them on your blog).