There are some mail clients that will block images by default. They may display a button on the email that says something like "Click here to download pictures." The user may be asked if they'd like to load the download images.
To allow the automatic downloading of images, try adding the the sender address to a safe senders list. To use Group Policy to set up the safe senders list in Outlook, please read the following article: https://support.microsoft.com/en-us/kb/2252421