If you discover that your recipients are not able to see your images, first look here. It could be that they simply need to allow, or show, or download the images within their specific email program.
When you're sure that your reader is allowing images in their email program, and the images still are not showing, it could be their virus protection or spam blocking software. Check with your reader to see if they (or their company's IT department, network, etc.) are using any kind of email protection software or service. A very common solution that will cause trouble with your email images is McAfee MXLogic Email Protection.
MXLogic uses something called an HTML Shield, which will change certain HTML tags in incoming emails. The whole email is likely to get through, but images are often replaced by a 1x1 pixel transparent gif image. This is most likely why only certain recipients are experiencing missing images. But nothing is actually wrong -- the Email Protection software is doing exactly what the settings are telling it to do.
You can see the various protection level settings for MXLogic, right here:
While folks probably won't want to change their Email Protection settings, it is possible for them to add sender IP addresses to their domain level Sender Allow list. Any email sent from those listed IP addresses would override the default blocks, and display the entire content of the email.
Unfortunately, there isn't anything we can do about images being replaced by the recipient's Email Protection software. This is due to a setting on their side, alone. But if you would like to provide an IP address for your readers to add to their Sender Allow list, check out these whitelisting instructions, here.
If you or your email system administrator require any assistance on how to change these settings, please visit the MXLogic Website, here.