If you see a broken image link in an eNotice where you haven't inserted an image, it's most likely the tracking image that alerts eNotify that a recipient has opened the message.
eNotices have a 1x1 pixel image in them that allows you to track the number of recipients who opened your eNotice. For more information about the tracking pixel, see Track email open rates.
In instances where an email client is set to not display images, all images in the email, including the tracking image, will display as broken. Allowing the images to display should resolve that issue.
If all the other images in the eNotice display, but the tracking image still displays as a broken image, it is probably due to a privacy setting in the recipient's email client or email server that suppresses tracking images. Adding the email address to the allowed senders list can cause the image not to display as broken. Obviously, this is within the control of only the recipient, so there is not a way to ensure that tracking images are not suppressed.
Outlook 2007 and Outlook Web Access (OWA) suppress tracking images (or "web beacons") by default. The web beacon suppression can be disabled for your own email by your email administrator, if desired.