Adding alt text varies depending on the email marketing platform you are using. Here are general steps for common platforms:
Mailchimp: In the content editor, click on the image block, then enter the alt text in the "Alt" field. Constant Contact: Select the image, click "Edit," and then add the alt text in the "Image Description" field. Campaign Monitor: Click on the image in the editor, then enter the alt text in the "Alt Text" field. HubSpot: In the email editor, click the image, then add the alt text in the "Alt Text" field.