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).
"When you get a message that has a picture in it, you see the picture automatically. "
Gmail automatically downloads and caches images, and loads them from their server instead of from the original remote server, when you read the email.
That only obfuscates the reader's IP. Gmail respects Cache-Control headers, so it will not cache the image if you tell it not to.
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.
Alpine, gui of sorts
K9, gui Android (think it blocks by default, or I turned off auto load when I installed ages ago).