Hi readinggal!
I sometimes add covers to my ebooks through Calibre. I only know how to do it when I'm converting the format (for instance going from lit to epub). First I look up the "cover" or picture I want in Google Images and save that image to my desktop. I try to find pics that aren't too small (anywhere from 250x400 or a little bigger seems to work well enough - at least for me). Then in Calibre when I start to convert a screen pops up to type in Title, Author, Output Format, etc. On that screen it gives you the option to "change cover image" so I just click on the "browse" button and find my pic on Desktop. It will show you the pic you selected and then you can hit ok to convert.
I have no idea how to add a cover without converting the format though. I have several ebooks already in epub that I want to change/add the cover but I don't know how to since I'm not converting the format. Maybe if someone else knows? If not in Calibre maybe some other program...
Anyway hope this helps a little. I annoys me when I buy an ebook and the cover is not there (even though it may be for legal reasons). I like to have the covers if possible - just like if I bought the printed copy.
Weezie