When sharing HTML data, a Share source app calls setHtmlFormat (in the DataPackage it gets in the eventArgs.request.data object) for the main markup and can then populate the package’s resourceMap with RandomAccessStreamReferences to local image files. This process is documented on How to share HTML.
What’s not documented at present (I’m filing a bug on this), are the allowable URI schemes when creating stream references with the RandomAccessStreamReference.createFromUri API can accept. Those are http, https, ms-appx, and ms-appdata. A small thing, but good to know!