What does ALT text mean?
Sometimes e-mail images are not rendered correctly. This is especially the case on narrow screens like mobile and toad. With an ALT text that briefly describes each image, you can at least convey the tank behind the image to those who can't see the image. Another positive attribute with using ALT text on images is that it increases delivery ability and reduces the chance that an email will end up in spam, especially for Gmail. It is clearly worth spending some extra time filling out this field when working with images in rule content editor.
To add alternative text to images:
-Double-click the image block you want to add alt text to, and in the field that says alternative text, change any text and Delete the image that is in the field per default.