When you add a image to an email you can add an Alt Image text. It is automatically set to default, but here you should write a text describing the image you are adding.
This is because the email provider might not show the images when you first open the email.
The text you have written behind each image will then be showed instead.
This usually happens when the email is marked as spam, unsafe or the email comes from an address you have never received emails from before. This will also help you increase the percentage of text in the email. Remember, each email should contain at least 25% text.