I believe there are a couple of approaches to this with varying amount of data storage, You can download all images immediately as the email is received (thus preventing image tracking as every image would then be clicked immediately) or have the ability to block all images and selectively turn them on when necessary (unless the image is inline etc).

Cool - not saying you have to do it like them, but the most popular option there is to block all images globally but allow the person to click a button to unblock images for that specific email, which then enables a dialogue to "always unblock images from this sender."

I like that workflow; may be something to consider. I block all images but allow images from known senders, like my parents or co-workers.