Because we designed the entire badge module as the link (the links works when you hover over the image or text), there's need for alternative text as long as your button text is descriptive. In fact, this module doesn't even offer an alternate text field. Consider these two examples:
If the images were removed in either of these badges, a sighted users would not be missing any content. As such, there's no need for any alternative text because the images provide no content not already covered by the text on the button.
In the second example where the image has text, it was the correct choice to make the button use the same text. Using "Learn More" instead would have been ambiguous and presented an accessibility issue.