The current version of the HTML5 spec on the img element here http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#the-img-element state :

Allowed ARIA role attribute values:
presentation role only, for an img element whose alt attribute's value is empty (alt=""), otherwise any role value.

The using WAI-ARIA in HTML, state in the recommendation table :
img with alt="" | role=presentation | NO | NONE 

So if i follow the HTML5 I can use role="presentation" only if the img has an alt="", and if I follow the recommendation table I can't.

One of the document must be changed at least.

I propose to change the recomandation table to :

img without alt | role="img" | NO | Role: Any, Any global aria-* attributes and any aria-* attributes applicable to the allowed roles

img with alt="" | role="presentation" | NO | role=presentation 

img with alt="some text" | role="img" | NO | Role: Any, Any global aria-* attributes and any aria-* attributes applicable to the allowed roles Not role=presentation

Otherwise the spec must change to something like :

Allowed ARIA role attribute values:
any role value except role presentation. 
(except if you want to allow it on img element with alt="some text" witch sound strange for me)