i've got a site where i want to randomize the branding image on the home page. not each time the user visits, but once per day

at the moment i'm thinking of using jquery to store a cookie when the user visits, which will last until midnight. then, if the user visits again that same day, just grab the image from the cache instead of sending them another random one

problem is, i've just heard of some strange EU rule on cookies, meaning you have to tell the user when you're using them. someone here said you don't have to tell the user / ask for permission / tick the popup box saying "can i use cookies" / etc IF the cookie is essential for site operation. but this is just to enable the caching of a simple image, albeit one that, from the site owners point of view, will massively cut down on bandwidth by not sending the users new images every five minutes

is it still ok to use cookies in this case? & if not, is there another way around this?

(i was thinking of maybe embedding a php script that automatically finds out which image should be shown on that day - & if that image is already in the cache just use that)