IMO , while Google does not penalize sites for using stock photographs on multipe domains, it's advisable to follow best practices for image optimization when adding them to your site for the most benefit. Some best practices and Matt Cutt's video link on the same issue below:
Matt Cutts on Stock Images
Image Optimization : Best Practices ( Important points highlighted below)
To help us index your images, make sure that:
- we can crawl both the HTML page the image is embedded in, and the image itself;
- the image is in one of our supported formats: BMP, GIF, JPEG, PNG, WebP or SVG.
Additionally, we recommend:
- that the image filename is related to the image’s content;
- that the alt attribute of the image describes the image in a human-friendly way;
- and finally, it also helps if the HTML page’s textual contents as well as the text near the image are related to the image.
Q: Should I really submit an Image Sitemap? What are the benefits?
A: Yes! Image Sitemaps help us learn about your new images and may also help us learn what the images are about.
Q: We sometimes see the original source of an image ranked lower than other sources; why is this?
A: Keep in mind that we use the textual content of a page when determining the context of an image. For example, if the original source is a page from an image gallery that has very little text, it can happen that a page with more textual context is chosen to be shown in search. If you feel you've identified very bad search results for a particular query, feel free to use the feedback link below the search results or to share your example in our Webmaster Help Forum.
Here's another useful article:
Image Publishing Guidelines
Cheers,
SEO5..
, well done.