Alternative text is added when the image is added to a Rich Text Box. Alt text provides an additional level of accessibility for viewers who may be using text-to-speech software to browse web pages.
To check or edit this feature select the Image. Then click on the Image icon on the toolbar of the rich text box.
The Alternative text can be added or edited in the Insert/Edit Image box that opens. Be sure to click Update, then Save and Close, when done.