Instead of using the meta
element, you must use the link
element (because the value is a URI):
<link itemprop="image" href="https://stackoverflow.com/questions/24638451/image.jpg" />
This is required by HTML5 (bold emphasis mine):
The
meta
element represents various kinds of metadata that cannot be expressed using the […]link
[…] elements.
And it’s also explicitly required by Microdata.