I gave it a try and it looks interesting, but I'm probably not part of your target audience. I don't know if it's a good or bad idea. I'll give you some usability feedback though based on my experience.

I put down a remark and a fake e-mail and was confused for a little bit. There was no submit button or any obvious way to inform the page that I was done entering info. I finally realised that I needed to press enter but this was not made clear to me.

The remark did not show up, but I noticed I was now logged in and ID'ed as my e-mail. I needed to enter the remark again before it showed anything.

I feel like it would have been more intuitive if the marks were visible even before my mouse hovered over the image. There can be an option to enable marks only on hover over but I don't think it's a good default. If your aim is to prevent the entire image from being covered in marks, then perhaps you could limit the number that show up (using some criteria like only the most recent X number of marks).