If you want to be sure that the image is always there, then use the standard watermarking feature - on-the-fly watermarking doesnt change the image itself and people can still link to the original if they are smart enough.
I use PNG-24 images with a transparency layer and upload the image to my server and then put that into the watermark image box. (There is an example watermark in the images directory: images/photopost.png).
Please do not PM me for support or sales questions. Thank you for your understanding.