ArcGIS Pro 2.6 API Reference Guide
PortalThumbnailUri Property
Example 

ArcGIS.Desktop.Core.Portal Namespace > PortalInfo Class : PortalThumbnailUri Property
Gets the URI of the portal thumbnail.
Syntax
public Uri PortalThumbnailUri {get;}
Public ReadOnly Property PortalThumbnailUri As Uri
Remarks
The portal uri does NOT include a token. If a token is required that should be added. The PortalThumbnailUri can be null
Example
PortalInfo portalInfo = await portal.GetPortalInfoAsync();
string thumbnailPath = portalInfo.PortalThumbnailUri?.AbsoluteUri;
if (portal.IsSignedOn() && !string.IsNullOrEmpty(thumbnailPath))
{
  var token = portal.GetToken();
  if (!string.IsNullOrEmpty(token))
  {
    thumbnailPath += "?token=" + token;
  }
}
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

PortalInfo Class
PortalInfo Members