With the way email clients behave there are two options:
Use images as attachments: with this option, images will always show up to your recipients even if they have images turned off but those images will be sent as email's attachments.
Use hosted images: with hosted images, the images won't be sent as the email's attachments but won't show up when the recipient has remote images turned off (they will be asked if they want to see images if they have turned off the images).
You can choose either of the two options.
Tip: We recommend using Option #2