Upload & Sell: On
The easiest way is to have a membership that allows you to upload images directly as part of the message editing. You can also upload images to other sites (i.e. Flicker, etc.) the code from them can be then be inserted (cut & paste) for the image into the message. I"m not sure @ code directly from your hd, as I've had a membership with Buy & Sell / Upload features for those.
HTH ... looking forward to seeing your work.