The button element is a container that allows both images and text on a button. Images must have alt-text as we discussed earlier. The following is a rather ugly example.
<button name="submit" type="submit" /> <strong>Go</strong> <img src="nextarrow.gif" alt="Next page" /> </button>
When the text carries the message of the button and the image is redundant, then alt="" is good for the image. In the example above, alt="next page" was chosen for the alt-text, so for assistive technology the button reads "go next page" which is more information than a sighted user would get.
When a button receives focus, assistive technology knows to announce the content of that button, including alternative text on image buttons.