In my Apple Mail you can enable/disable the automatic download of images in HTML messages. In Mail Preferences, select the Viewing icon/options and in there should be a check box that says "Display Remote images in HTML messages". If you disable the images, the message is displayed with "placeholders" (small question mark" but the message also includes a new banner with a "Load Images" button (so if you want to see the images you just press that button and images are downloaded for that message only (i.e. does not change the preference setting).
Stuart
Stuart