There's been some talk on PR lately of images, and companies and/or people using them without permission, and it's come up that a lot of people (including dfr!) don't know how to watermark their images. The thread that prompted this post was the one started by Rebecca of Tales of a Wannabe Seamstress about pattern companies publishing blog photos on Facebook, but there have been several other threads recently on similar topics.
While someone with much photo-editing skill can probably remove most watermarks, just having it decreases the temptation to simply swipe an image because it's easy, and increases the likelihood that if someone else does use your images, they'll at least credit you.
In general, circulation of your work through comments, links, excerpts, etc., is usually positive, and inclusion on gallery and "social collection" sites like Pinterest or Wists is generally a good thing. Whenever you put something up online, you shouldn't have the expectation that it will be private, and usually visibility is what you want. But having your images simply stolen is never fun.
You should consider carefully before objecting to properly attributed excerpts, even if you weren't asked. Interlinking and commenting on what other people have done is what makes the blogosphere go 'round. If someone has quoted you or shown a reasonable picture or screenshot of something you've done that could be considered fair use, and has linked to you and properly credited it, you should probably be happy rather than angry. The guideline on fair use is that their use shouldn't decrease the value of what you've published. That doesn't mean they can't criticize it, it just means that they can't include so much that your own post would become redundant to their readers. The legal criteria for fair use are outlined by United States Code Title 17, Subsection 107.
How To Watermark Your Images
So, some quick instructions on watermarking. Most of what you're going to do uses the Layers/Channels/Paths/Undo dialog. It should open along with GIMP, but if it dosn't, you can access it through the "windows" menu in GIMP's top toolbar. (You can view the larger versions of images in this tutorial, and on my blog in general, by clicking on them.)
What you're going to want to do is create a new layer for the watermark.
Note that when you add text, that text is always added in a new layer, and the layer is automatically labeled by what the text says. So if you're watermarking using text only, you shouldn't need to add your new layer manually.
New layers added using the "add new layer" button (Circled in red) are by default called "New layer," "New layer #1," etc. If you right click on the layer in the layers dialog (Not in your image!) the first option is "Layer attributes," which will allow you to change the name of your layer if you wish to do so.
Once you've created a new layer, whether it's text or an image (I've created a GIMP brush for my watermark, so I can easily stamp it anywhere) you can manipulate the layer opacity and mode using the options just above the list of layers. Whatever layer is selected will be the one affected by your selections. Opacity should be fairly self explanatory: 0 is completely transparent (invisible) and 100 is completely opaque.
The layer modes contain things like overlay, darken only, lighten only, and value. They dictate how the layer shows itself. For example, if you make a layer the "darken only" mode, you'll only see the contents of that layer in places where the selected layer is darker than what is under it.
When you select Edit > Paste as > Paste as New Layer, your newly pasted layer will be a floating selection, not yet anchored to a "real" layer. You can fix this by clicking the "create new layer" button. Otherwise, if you paste anything else, that floating section will automatically anchor to the layer beneath it. I believe anchoring is the same as merging down. It's not the same as moving down, which is simply rearranging the order of the layers.