Image Challenge suggestions appear to be generally listed in order of posting, with the newest responses first. However, whilst a question is open, other users can click "I like this!" which gives a score of one point to that answer. Once a question is closed answers are listed by the number of users who have clicked the "I like this!" on that answer; however, as many answers are submitted each week, most are never clicked on and so are sorted by the time they were posted.