To add a watermark, simply open the image in your graphics editor, add text to bottom of image, save and upload.

If you have one of the more upmarket graphics editors e.g PhotoShop, FireWorks etc you can do all your images in a batch. Put all your images in a folder on your PC, set up the batch job to add the watermark and run. You'll be able to do all your images in a matter of minutes.

For more specific instructions on how to batch process simply Google something similar to "photoshop batch processing".

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Lawrence