There does not appear to be a single markdown construct that will do what you want, however both of the following pieces of markdown work (as least on SO):
I agree it would be a handy thing to have (along with markdown markup for width, height, floating left or right and adding a caption).
EDIT: I don't think there is any markdown available for the
rel attribute of links.
<a href="http://google.com.au/" rel="some text">![Foo](http://www.google.com.au/images/nav_logo7.png)</a>
The above looks like your best case, although by this stage you almost might as well just use HTML for the image as well and be done with it.