Alternative text (properties of an image, located in the Edit link) is not the same as hover text or a tooltip.
Alt text is meant to be a description of the images. This text will display in your visitor's browser if your image fails to load. It also assists the search engines in providing accurate search results on images searches.
Search engines may penalize you for not having Alt text, so it is a good practice to add this for all images via Edit link on your Picture widget.