SSRS using external images through tls 1.2 leaves src(unknown)
I ran into this issue when attempting to run images from IIS Server which was locked down to TLS1.1 and TLS1.2

The issue was two fold

I had to update the Registry to add the following keys

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727]
"SystemDefaultTlsVersions"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727]
"SystemDefaultTlsVersions"=dword:00000001 


per this article https://support.microsoft.com/en-us/help/3154519/support-for-tls-system-default-versions-included-in-the--net-framework

I also had to use the following Entry in the configuration file for Reporting Services 


  [Install Location]/Reporting
  Services/Bin/ReportingServicesService.exe.conf
      inside the runtime element
  
  Default SQL Server 2016 Location: "C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\ReportServer\bin\ReportingServicesService.exe.config"


&lt;AppContextSwitchOverrides value="Switch.System.ServiceModel.DisableUsingServicePointManagerSecurityProtocols=false;Switch.System.Net.DontEnableSchUseStrongCrypto=false" /&gt;


After all of this all I had to do is restart the reporting service and it was fine

Note that i do have tls 1.1 and 1.2 locked down my SSRS and SQL Server (not sure if this is also a requirement for this fix)

My System Sql Server 2016 and Windows Server 2012 R2