If you are already using GIMP and Adobe Photoshop, then uMark is not for you as it does not offer any additional functionality.
Why might you want to place a watermark on your images? You would if you wanted to protect them. Umark makes it difficult for someone to steal an image without others seeing that they do not own the rights to it. Of course, watermarks are not foolproof as there advanced users who know how to remove them.
There is both a free "lite" version and a professional version of the application. The free version has some built in limitations on placement, position and text. It also adds the "uMark Lite" notation to each image in addition to the watermark that you create.
Example of image with watermark:
Such restrictions don't have any great effect on the usability of the software, but one limitation that may be significant to some users is that uMark Lite does not preserve EXIF information in images. This does mean that embedded image owner and copyright information may be stripped as a result of applying a watermark, though it should be noted that this does not happen with the Professional version.
The application is available on both Windows and Mac and can be downloaded at http://www.uconomix.com/.
Below is a video demonstrating the application:
You get a free, 15-day trial when you purchase the Professional version. Check it out.