[XHTML 2] 19.1 the img element

Dear HTML Working Group,

What does "not presentable" mean in the following sentence of 19.1

"Like the object element, this element's content is only presented if the 
referenced resource is not presentable."

Please clarify this to say exactly what cases an image may be not 
presentable, for example:

<img src="chicken" type="text/ecmascript">Fallback!</img>

A javascript resource has no presentation, so is not presentable, therefore 
the fallback should be used, or not?

Please specify this in a manner identical to other fallback behaviour in 
Section 17, although please ensure this is clarified first.

Regards,

Jim Ley