Every day Flickr uses an "interestingness" algorithm to automatically select 500 "interesting" images from the millions that are uploaded, and they are featured for 24 hours on that day's Explore page. There are lots of elements that make something "interesting" (or not) on Flickr. Where the clickthroughs are coming from; who comments on it and when; who marks it as a favorite; its tags and many more things which are constantly changing. Interestingness changes over time. You can see the images I've posted on Flickr that have been included in the daily Explore pages below.