There have been some known bugs with NN and Alt Tags. One of them has to do when surfers are browsing with their images turned off. If you do not have image size specified, the user will not see the alt tags, bummer!
I've been testing NN6.2 and it appears that it is not showing alt tags. It does show the a title tag which NN4+ did not.
I think they may have some bugs to work out.