Custom Images in MS Visio

[TDills]

Adding custom images to a Visio file fail when activating the custom tab.
The error code provide is:
Custom UI RUntime Error in "...."
No loadImage callback was specified to load the image with id "..."

Running the exact same code in Excel documents work beautifully (without manually specifying loadimage callback), so it seems to be specific to Visio.

To Reproduce
Steps to reproduce the behavior:

1. Add a custom image to a button (in MS Visio!)
2. Activate the Tab in question

Expected behavior
The custom image should appear as an icon in the ribbon, just like it does in Excel.

Screenshots
See error code

Additional context
Ribbon Code: