I made a small user script that lets you use the back button to exit the image overlay! If you already have Tampermonkey or friends installed, click here to install it!
After installing hitting the back button while viewing an image in the overlay should close the image instead of going back a full page.
It mucks with the page history stack to make itself work; if you encounter any issues with the back button not working as you expect, let me know by commenting or filing an issue on Github.
