Although WAI-ARIA attributes fall first in the accessible name calculation for img elements according to the HTML to Platform Accessibility APIs Implementation Guide, it is always best to use the most robust attribute: alt.
VoiceOver for iOS and OS X allows the user to choose an option to always, never, or only navigate images with descriptions (accessible names). VoiceOver reads the file name of the image with no accessible name by default. NVDA & JAWS does not read the file name of the image with no accessible name at all, the user would not know it is present on the page. On Android in Chrome the image with no name is ignored, in Firefox the image with no name is read as "graphic".
Pennybacker Bridge via aria-labelledby