Invisible watermark on attached images

[PlymWS]

I don't know if this mod is / would be available but here goes ..

My site contains images which the uploader owns. We have suffered a couple of instances where these images have been posted elsewhere.

Is a mod available that will somehoe attach my sites name and the userid of the person downloading the image that can only be retrieved with a code or similar. I would like this to be invisible so as not to spoil the image, but allow me to find ( and ban ) users who post images elsewhere ?

[nexialys]

if you look at the Similar Threads of your post, you will have your answer...

[PlymWS]

Yes, but they only print another image on the original image. I would like to print the userid of the person downloading the image instead of a png.