Gemail, Outlook.com/hotmail, thunderbird and outlook all block images, unless whitelisted... Now, there may be other providers/clients that don't, but the above accounts for a significant number of users (if not most western mail users).
Gmail does something more interesting. It shows the image by default if the url is not unique, or the image is an attachment. It also uses googles servers to download and cache the image when receiving it.
