Store a picture in a cookie

Store it in localStorage using JavaScript.

https://web.archive.org/web/20160313121817/https://www.ibm.com/developerworks/mydeveloperworks/blogs/bobleah/entry/html5_code_example_store_images_using_localstorage57?lang=en

Surely it's more efficent to store a link to an online image in the cookie, and use that instead?

Each time a webpage is browsed, the cookies get transmitted - storing an image will add unecessary bandwidth.