To do this with PowerPress:
1.) Log into your WordPress site
2.) Go to PowerPress >> Settings, Advanced Mode >> Episodes
3.) Check the box next to iTunes Episode Image Field and save the changes
4.) Now, you'll have a field within the Podcast Episode box of your posts called iTunes Image, just copy/paste the URL to that episode's image from wherever you uploaded it to, and the image will be added to your podcast RSS feed
As far as embedding the artwork image in your episodes' ID3 tags, that depends on how you're handling the tags. How are you doing that now?