Full credit for this tip to Andrew Sarratori from Microsoft who was helping me out on Gitter. Thanks Andrew!

Building Visual Studio extensions still feels like blundering around in the dark sometimes. One such example of this is icons and image resources. After icons from an extension failed to load, I needed some way to debug my issues.

Fortunately, Visual Studio has this built-in thanks to the seemingly undocumented "ImageLibrary" registry key. You can turn on ImageService logging by adding the following keys:

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0\ImageLibrary]
"TraceFilename"="ImageLibrary.log"
"TraceLevel"="Verbose"

Now start Visual Studio and you will see a new ImageLibrary.log file show up in your user folder with crazy amounts of detail as to how Visual Studio is reading and loading your image resources.

To apply this to your experimental instance instead, just add the same keys to the HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0Exp\ImageLibrary path instead (where Exp is your experimental suffix).