Maybe someone finds this question while searching for "how to use Schema.org ImageObject", as I did, so, just for reference, here is an example from the Schema.org documentation:
Without markup:
<h2>Beach in Mexico</h2>
<img src="mexico-beach.jpg" alt="Sunny, sandy beach."/>
By Jane Doe
Photographed in Puerto Vallarta, Mexico
Date uploaded: Jan 25, 2008
I took this picture while on vacation last year.
Microdata:
<div itemscope itemtype="http://schema.org/ImageObject">
<h2 itemprop="name">Beach in Mexico</h2>
<img src="mexico-beach.jpg"
alt="Sunny, sandy beach."
itemprop="contentUrl" />
By <span itemprop="author">Jane Doe</span>
Photographed in
<span itemprop="contentLocation">Puerto Vallarta, Mexico</span>
Date uploaded:
<meta itemprop="datePublished" content="2008-01-25">Jan 25, 2008
<span itemprop="description">I took this picture while on vacation last year.</span>
</div>
http://schema.org/ImageObject#examples