In non-beta when images are disabled images are replaced with a link and alt text like so (on Barack Obama article):
<a href="/w/index.php/File:Official_portrait_of_Barack_Obama.jpg" class="image"><span class="mw-mf-image-replacement">[A portrait shot of Barack Obama, looking straight ahead. He has short black hair, and is wearing a dark navy blazer with a blue striped tie over a light blue collared shirt. In the background are two flags hanging from separate flagpoles: the American flag, and the flag of the Executive Office of the President.]</span></a>
The api doesn't replace the images like this - it simply strips it altogether. The API should be consistent with the mobile site rendering.