The ArcGIS REST API documentation indicates that the thumbnail image for a group can be updated by calling the /update endpoint on the group and passing in a URL (either absolute or relative) to the image to use. There are 2 issues with this:
If I update the logo via Portal, I can see that the image is embedded in the request payload. This is ideally what I'd like to do.
I'm creating an HttpRequestMessage in C#, so how would I go about properly embedding the image in the request payload?