Image alt tags are a crucial part of any websites SEO. By providing your images with an alt tag, your site will score better with search engines such as Google and subsequently rank higher in the search results.
To edit the Alt Tag of an image, simply click on the image you want to edit and below the image preview will be an "Image Alt Tag" field where you can put whatever you'd like:
Please Note: Once an Alt Tag has been modified, you will need to re-publish BOTH your files and the web pages with the image displayed.
Guides for both can be found here: