I have created a short (okay, 15 minutes… so not that short) step-by-step video on how to make your own watermark in Paint.net and then place it on a photo. If you are receiving this post via email feed, the video won't show up, you must go back to the article (click on article title).
I have created two previous videos on how to download free Paint.net and install it as well as how to edit a photograph and embed a watermark in it HERE. The latter video assumes you already have a watermark and this one shows you how to create and save as a transparent .png. Good luck!
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