I will weigh in with my own two cents as a serious amateur photographer who uses photos on blogs and forums almost daily, and who receives requests to use his photographic work on occasion...
Firstly, exactly 99.9999% of all photos taken by amateurs to accompany blog or forum posts has zero monetary value outside of the aggregate value to the forum owner and the knowledge value to the reader. That means no one really wants to steal your work and reuse it to make money.
Watermarking can take time if you are amateur who doesn't have automated watermark process, and often watermarks can obscure what you want the photo to illustrate, so why bother? I'd expect most readers here who want to use your image on their own site to help illustrate a technique are likely to ask your permission anyway... this is a close community.
Personally, I do not watermark my images that I post to the typical forums or blogs or restrict others from using my images... I believe in shared knowledge and ideas which is the purpose of sites such as this.
Only rarely when I post images as part of a specific commercial project, or if I post a studio type professional image of a model do I restrict copyright. In those cases I mostly desire the image attribution when it is used, but sometimes it is necessary to ensure that if an image is marketable that I share in the monetization of the image. So sometimes I use a creative commons license and copyright notice included in the image, other times I use a fullblown restricted copyrigh, but again only in special cases.
In those cases I use Photoshop to apply my watermark layer and I use digimarc service and filter to embed digital rights information in the file which may be tracked on the internet as well.
If you want to add watermark, find any image editing software and add text using a separate image layer with your copyright symbol year and name. Use either white or black text to contrast with the background image. Set layer opacity between 50 to 75%. Position layer where you like... (such as lower left corner) and save image. The image itself is not damaged using the layers technique. In my attached photo I used Photoshop to add two separate text layers as an example... When I send proofs to someone I may enlarge and place the copyright diagonally over the image so the recipient doesn't try to use these images as the final image... unedited low res images print like crap.
It is easiest to use picasa... makes posting images easy too.
Adding watermarks to photos
You can help protect your images by adding a watermark to your photos. A watermark in Picasa is white text that's added to the lower-right corner of photos during the export or upload process. To add a watermark to a photo, please use the following steps:
During export
Select the photos you'd like to export.
Click the Export button in the Photo Tray.
Select the 'Add a watermark' checkbox.
In the watermark box, enter your watermark text.
Click OK. Your photos will be exported with your watermark in place.
While uploading
Before you upload, click Tools > Options (Windows) or Picasa > Preferences (Mac).
Select the Picasa Web Albums or Google+ Photos tab.
Select the 'Add a watermark for all uploads' checkbox.
In the watermark box, enter your watermark text.
Click OK. All uploaded photos will possess your chosen watermark.
Picasa will never make changes to your original photo file. Exporting and uploading pictures with edits or watermarks applied creates an edited JPG copy of the original photo.
You can see how the one copyright notice obscures the grooms face making it next to impossible for the bride and groom to view the image to select printed copies. It could just as easily have been the interface between the work piece and cutter that was obscured... On the other hand the lower right copyright could easily be cropped out of the image if someone wanted.... Even with copyright infringement you need to know about it, and prove damages and that the person benefited financially if you wish to sue for any real compensation... I guarantee that no one here can afford the costs of enforcement. So again, why bother.
