What is the recommended image to text ratio in a message for best deliverability?

This depends on sender reputation and providing the best end-user experience. The rule of thumb is 60/40 or 70/30 but this is just one factor in reputation. There is not an exact formula that works effectively under all circumstances, so you should test. Less advanced spam filters pick up all images with no text as potentially spam as spammers have used images to hide keywords and spammy content. The user experience should dictate enough text and/or alt tags so that the user should be able to view message content without turning images on.

• Strive for a good balance of text and images in a way that makes sense for your business.
• Use enough text in the body of the message so subscribers get a sense of the message content whether images are on or off.
• Favor a heavier use of images for emails targeted at a mobile viewership, since they make great tap targets.
• Test emails to see how it performs to ensure your message is inboxing.