To be able to do this in a simple way, you can use a plug-in for Lightroom called LR/Mogrify. Timothy Armes is the developer of this great plug-in and apart from for the graphical watermark function you can do much more funny things; You can create borders, Text annotations, resize photos and sharpen them, modify color space and play with colors’ processing.
All effects are applied on export and each one of them can easily be customized to your needs.

The free version of this plug-in can export 10 images at a time but a donation of any amount will give you an unrestricted version of this software.

Here you can download the plug-in and also read the installation instructions:
»Get the Plug-in!
Be sure to download the right version of the plug-in since it’s available for both LR 1 and LR 2.

! To get this to work you also need to download ImageMagick ( Currently I have version “ImageMagick-6.4.4-Q16) which is the image engine used with this plugin: http://www.imagemagick.org

This is how I use the plug-in to put image stamps on my photos:

1. I start by making the watermark stamp in Photoshop; To be sure my stamp doesn’t get placed to close the photos corners I put 10px of margin around it. In Photoshop I make the background layer invisible and put a stroke effect with 2px on my text/logo layer. I prefer to use white color for my stamp (text) since it looks good on most of my photos. The black stroke around it makes sure it will be visible on really light photos as well.
2. To get the feeling of a soft semi-transparent stamp, I change the Opacity of my text/logo layer to 50%.
3. When I’m satisfied I save my logo as a PNG image which is a great format that can handle transparent photos with anti-aliasing edges (GIF is not an option for me here).
4. When saved, the stamp is ready for Lightroom.
5. When I have installed the LR2/Mogrify plug-in to Lightroom I simply open the Export window as usual. A few clicks are needed so I start by inserting “Mogrify Configuration” and “Graphical Watermark” from the “Post-Process Actions:”-panel (you find it on the lower left side in the export window).
6. Since this plug-in is using open source software called ImageMagic to modify the images, the path to the software needs to be added. I add my installation path of Mogrify.exe to the Mogrify Configuration panel (See, “Path of Mogrify application:” ).
7. In the “Mogrify Graphical Watermark” panel I then add the path to my stamp/logo.
8. Finally I select the position for my stamp by using the checkbox option for “Position”. This selection will place my stamp in the bottom right corner.

Instruction steps 1-4

Instruction steps 5-8

My final image exported with a stamp directly from Lightroom:

See more of Timothys work and other plugins here.