ALT texts for images are important. When an image is not loaded by default by an e-mail client, the reader can still see the message of your image. Speech computers also read the ALT text, which is convenient for people with a visual impairment.
You adjust the ALT text in the properties of the image when you place it in a message.
Click on the three dots by the image and the following pop-up will appear:
This is the place where you can add an ALT text to your image.