Creating an ImageSource resource like this works fine:

<ImageSource x:Key="Image.DNA">pack

if I use a tool like XAMLStyle to reformat the code, which ends up looking like this:

<ImageSource

x:Key="Image.DNA">

pack://application:,,,/HelloWorld;component/Assets/NoesisGUI/HelloWorld/Resources/Images/DNA.jpg

</ImageSource>

I get this:

ArgumentException: Illegal characters in path.

System.IO.Path.IsPathRooted (System.String path) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

System.IO.Path.InsecureGetFullPath (System.String path) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

System.IO.Path.GetFullPath (System.String path) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

NoesisPostprocessor.NormalizePath (System.String uri) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:124)

NoesisPostprocessor.AbsolutePath (System.String parent, System.String uri) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:152)

NoesisPostprocessor.ScanTexture (System.String directory, System.String text, System.String extension, System.Collections.Generic.HashSet`1[T] textures) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:201)

NoesisPostprocessor.ScanTextures (NoesisXaml xaml, System.String directory, System.String text) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:220)

NoesisPostprocessor.ScanDependencies (NoesisXaml xaml, System.String directory) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:311)

NoesisPostprocessor.ImportXaml (NoesisXaml xaml, System.String directory, System.IO.FileStream file) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:323)

NoesisPostprocessor.ImportXaml (System.String filename) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:336)

NoesisPostprocessor.ImportAssets (System.String[] assets, NoesisPostprocessor+UpdateProgress d) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:67)

NoesisPostprocessor.OnPostprocessAllAssets (System.String[] importedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromAssetPaths) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:82)

System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.

System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <f2e6809acb14476a81f399aeb800f8f2>:0)

UnityEditor.AssetPostprocessingInternal.PostprocessAllAssets (System.String[] importedAssets, System.String[] addedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromPathAssets) (at C:/buildslave/unity/build/Editor/Mono/AssetPostprocessor.cs:140)