<PackageReference Include="Magick.NET-Q8-AnyCPU" Version="7.21.0" />
<PackageReference Include="Magick.NET.SystemWindowsMedia" Version="2.0.0" />
Taking the title into consideration, one can't use Magick.NET.SystemWindowsMedia in a .netcoreapp since, the package is not being used, despite it being installed.
You can take a look at the public meta data via fuget
The result of the package not being used or deployed is, that the extension method for turning a MagickImage into a BitmapSource isnt available.

You can take a look at my previous bug report #683 if you want to validate this yourself. The screenshot was taking from the MvvmScarletToolkit.Samples project here.
Screenshot of the debug output, validating that the required dll isnt being deployed:

Edit: did download and extract the .nuget file.
there is no folder for .netcoreapp which would contain the appropriate dlls. Only the .net40 stuff is present.

Thanks for reporting this. Turns out that I forgot to add the netcoreapp3.0 files to the package. That will be resolved in the next release that I will try to publish this week.
The new version has been published.
Tested the new version. Its working now. Thanks! :)
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Thanks for reporting this. Turns out that I forgot to add the netcoreapp3.0 files to the package. That will be resolved in the next release that I will try to publish this week.