This issue is a common problem when sending an email with CID (Content-ID) embedded images. These images can greatly increase the size of the email after it's sent (especially if the email has multiple images). The email usually displays correctly in desktop email clients, but often cause issues when viewed with a mobile client. The increased size causes the email to download in Plain Text (rather than HTML) when a recipient opens it on a mobile device, preventing the images from being displayed.
In these instances, the top of the email indicates "This message was downloaded as plain text." However, directly below that message is a "Download full message" link. Clicking on that link should restore the email to HTML format and the images should then display as normal.