This is the bare minimum, and it will serve the purpose nicely. Personally, I like to send an email which congratulates the winner and says some good things about his site. For example, I might mention that the page about their pet was especially cute, the article on how they survived some disease was meaningful or the gardening tips were especially useful. The point is simple - by including such details I'm showing that I did actually look at the site, which makes the award more meaningful. Yes, I know that stationary looks great, but not everyone can read them.
All email clients can read text messages.
It's a good idea to attach the graphic directly to email message, as well as to include a link where it can be picked up if, for some reason, the image is not received properly. I've found an email similar to below hits upon all of the necessary points. Hi Sally, Thanks for applying for the Excellent Site award. I found your site is indeed excellent and I thoroughly enjoyed my visit. Your award image is attached.