Hi everyone!

I'm currently learning how to build chrome extensions, and I'm stuck.

I want to have a basic extension that shows a series of images, one image each day, depending on the number of days the user installed the extension.
For example, when a user installs the extension, he will see image 1. 2 Days later he will see image 3, etc.
My question is how to implement this? My current guess is to use some sort of tracking cookies but I'm not sure how.
Pointers and/or code samples would be great.
Thanks! Dvir