I changed mine about a week ago and they did change (I think it took 12-24 hours to see the change). There are bugs in collections though - if you change the cover image it appears never to show up if that collection is one of the featured in your portfolio.

I don't know if it's important to buyers (I'm assuming it isn't) and there are so many other things I'd prefer developers to focus on if they do at some point start to work on the contributor interface (stats on image earnings; searching one's portfolio by keyword on the contributor side, automatic monthly payments,  . . . ) that I'm not inclined to report the cosmetic problems