Watermarks are the best option to save the illegal circulation of pictures and they can be easily added by photo editing softwares like Photoshop or Corel but in case you want to do it online then there is a site Picmarkr which enables to add watermarks online by just uploading the pic.

Here is a quick procedure for this:

1) Upload your pics maximum of 5 files and 25 Mb and choose image width size i.e. 500,800,1024 pixels

2) Then choose from Simple Text/Image/Tilted

After uploading you can download the final image or export to Flickr account. I find this utility as a good option for online usage but the restricted image size and no color for text watermarks are downfalls of this application.