Some clients complain they can't see the images inside the communications I send to them. What would be the explanation?

This is usually due to the fact that email services providers (like Gmail or Yahoo! Mail) strip the messages of their images and leave their display as an option offered to users. Tell your clients to look for a link in their email that they need to click on to see the images included in the message. It would be a reminder like "This message contains blocked images" near a button/link that says "Show Images".