Katy. The alt attribute shown upon mouseover is only functional in IE. It does not appear upon mouseover in other browsers such as Firefox. Hopefully, your learners will only be using IE to view your courseware.

Also, according to 1194.22, the purpose of an alt tag is to provide a text equivalent for every non-text element. In this case, the alt-tag is read by the assistive technology, such as a screen reader, which describes the image to a visually impared learner; who would not see the mouseover anyway.

To address your query directly, you would indeed need to edit the module's xml file where the images appear. As an example, this would be a straight forward alt="Image of a person speaking."