Hi Donald,
Thank you for writing to Microsoft Community forum.
Windows uses Miracast for mirroring contents on other Miracast compatible devices. As of now, we cannot directly cast the pictures using Chromecast. Instead, you may cast the desktop and open Photos app to cast pictures.
As per my research, there are a few apps on the Store which can help you cast local/native pictures using Chromecast. You may search for an app in the Microsoft Store which supports this feature and install it.
For Windows to support this feature, I suggest you to post a feedback in the Feedback Hub, so that our Developer team can have a look into your suggestion and include the feature in the future updates.
Regards,