Hi,

There is a module available in Odoo apps store that watermark the product images. 

Here is the link: https://www.odoo.com/apps/modules/10.0/product_watermarking/

Gravedigging but for a purpose: now that Odoo has a fully functioning (ehem) e-commerce module, would be very nice to have!

I am also interested in such feature when uploading image to website. I have tried to inherit the 'website.editor.dialog.image' to add an element in dropdown menu when selecting image upload, but without any success. I don't have much experience in modifying Odoo's apps and widgets.

e.g. A company is creating its portfolio with photos from the projects done. There are stealing companies using those photos as their. If the photos are watermarked they are unusable outside the owner's company website.