The problem became apparent if you wanted to use something like a Roll Up Image on content pages where you wanted to display the image both using Content Query Web Parts (which don't have a problem with Publishing Image fields) and in Search Results. The problem is, Search doesn't index fields of this type. It just doesn't. So you can set up your metadata property which you can add a property mapping for the Publishing Rollup Image field but when you try to surface the data contained by the property in search it contains nothing.
Having experienced this problem in 2007 during a custom Search project, I was already aware of the limitation and then SharePoint 2010 came around and a similar requirement came about. I was approached to take a look into whether it was still a problem and had some expectation that perhaps it had been resolved. Alas, it has not.
So, buyer beware!!
How To Resolve
Fortunately, there is a simple work-around. Create a text field into which you automatically add the contents from your Publishing Image field as text by whatever means you prefer and then use that to set up your metadata property instead. Works like a charm.