Custom UI Editor - Built-in Images

These are static as it happens at design-time.
There are several different ways an image can be loaded dynamically:

You can use ImageMso to assign a built-in icon to your controls

<button id="Button1" 
        imageMso="HappyFace" 
        size="large" 
        label="Large Button" 
        onAction="ThisWorkbook.MyMacro"/>