Branded Ribbon with Custom PNG Icons

Problem Description

Meridian Wealth Management wants their PyXLL Excel add-in to display their corporate branding in the ribbon. Their design team has produced two custom button icons as PNG files with alpha-channel transparency. The icons are shipped as resources inside their Python package mw_addin:

• run_analysis.png — icon for the "Run Analysis" button
• export_report.png — icon for the "Export Report" button

A third button ("Refresh Data") should use a standard built-in Office icon rather than a custom one.

The PNGs must render with correct transparency — earlier attempts using the default image loading produced black backgrounds instead of transparent ones.

All callbacks live in the mw_addin.ribbon module.

Output Specification

Produce mw_ribbon.xml — a complete customUI XML file with a ribbon tab containing the three buttons described above, with images configured correctly for each.