To protect users of email software like Outlook, images are not downloaded by default. They are downloaded immediately by Gmail, but users can actually choose if they want to download them or not. According to research by Litmus, 43% of Gmail users actually do not download the images by default.
Among email clients, the most important reason for this is security; among individual persons, it is bandwidth. To download images, the recipient has to click.
This is not necessarily the end of the world for your email communicationMore information is provided in this article.