What I can tell you for certain, is that adding an image sitemap into your webmaster account, allows you to tracking the index status of your images.
For example, lets say I have some 10 odd pages on my site, each with 2-3 images on them. That's around 30 images overall linked to the actual pages. However, when I was uploading those images, I uploaded 50+ images. Clearly, some of these images are not linked on the visible site.
What an image sitemap will allow you to do is give you a simple status such as 20 images out of your 50 images have been indexed. This should address your question.