If it isn't immediately obvious when you look at the mobi covers that they've been downsampled - if that doesn't jump out at you or bother you - then you win! No reason to look for them in a different format.
It seems pretty obvious to me, though, and the epub cover image looks much clearer and sharper. You seriously don't see any difference in the clarity between
http://farm4.static.flickr.com/3646/...7f4958f8_o.jpg and
http://farm4.static.flickr.com/3579/...a9f7819b_o.jpg ?