your report should include the page names that have the empty alt tags. Using that information, you should be be able to edit those pages, find any empty alt tags and edit the code.
alternatively, you could use Dreamweaver Site Search to search for all instances of
and edit each one that is found.
as for the duplicate description and keyword meta tags, I am not able to reproduce this in my version of power store on the index page, can you send a link to your site where i can investigate.