Chris Nolan has some good advice for using the <image> element in RSS.
If you’re going to use a channel image and you want it to be consistently displayed put in those optional fields with your image’s true dimensions.
And he points out that Rmail enforces the default image height and width causing some images to be distorted including that of The RSS Blog. Note also that Rmail also enforces the maximum image height and width values.
Robyn Tippins
Sleepyblogger.com