If you link to an image from an email (with an <img> tag) then: * It looks like you're tracking which recipients read your email and when. Spammers do this a l

If you link to an image from an email (with an <img> tag) then: * It looks like you're tracking which recipients read your email and when. Spammers do this a lot (or rather they used to) with single-pixel images called "tracking beacons". You send an email to millions of people, give each one a slightly different <img> link, look at the server log to see which images got downloaded, and those are your suckers. * Because of the previous point, most email clients don't load images from external sources by default. They pop up a little notification bar saying something scary like "Click here to load unsafe images", because "Click here to load external images that might cause spammers to see whether you've looked at the email" is too complex. So your emails all now come with a scary warning, and most people won't see your image anyway.