Images overlay

Not sure if I can explain this correctly 🙂

I’m looking for a way to implement “image overlays” on post images. Something like a top layer / div on the image itself (maybe a .png image with a magnf. glass or something similar, hope you get the idea :).

I can code it manually to a template just fine, but does any one know a way for a automatic solution for post images? Does it require core hacks?