I tested this in Ubuntu 16.10 and it works beautifully. The effect is very subtle, so it doesn't overwhelm the artwork, but I think it would be difficult to remove unless you were very determined. Thanks for making this available, Redrobes.

I was going to upload my pic to show my results, but it won't let me! (I believe permissions on this folder are very limited and, for example, regular users can't reply to you). Too bad. Although I'm not personally concerned with copyright, I think this tool is worth a look for those who are.