Google has added support for image credits for image metadata. Previously, image credit information was only supported with IPTC photo metadata. Google wrote "Added support for image credits to the Image Metadata structured data documentation. Previously, you could only provide image credit information with IPTC photo metadata."
You can see this help document now talks about image credit throughout the document. The previous version had no mention of image credit for this metadata type.
It now says "When you specify image metadata, Google Images can show more details about the image, such as who the creator is, how people can use an image, and credit information." CreditText is the name of person and/or organization that is credited for the image when it's published.
Here is sample code:
Forum discussion at Twitter.
