Selected answers from the Dumb SEO Questions G+ community.
Casey Markee: Are the images hosted on a CDN? That`s usually the first thing that can make it harder for Google find and index those images.
On Wordpress, if you are using Jetpack and their Photon image CDN, Google basically doesn`t index ANY of the images. You have to take extra steps there or just "turn it off."
Rob Woods: that`s the case with almost every image sitemap I look at. I wouldn`t worry about it