ArcGIS Pro 2.8 API Reference Guide
SmallImage Property (PlugIn)
Example 

ArcGIS.Desktop.Framework.Contracts Namespace > PlugIn Class : SmallImage Property
Gets or sets the small representation of the command, this may be an image or a xaml element.
Syntax
public object SmallImage {get; set;}
Public Property SmallImage As Object
Remarks

Many ribbon controls have three size definitions: large, middle, and small. The large representation is a 32x32 image over its caption. Middle is a 16x16 image beside its caption and small is a 16x16 image only. You should always supply both a large and small image as these may automatically swap depending on the available space in the ribbon.

This property will by default return the small image specified in its DAML declaration. Use this property if you need to change the small image at run-time.

Example
<button id="acme_rename" caption="Rename" className="RenameProjectItem" smallImage="/Images/GenericPencil16.png">
  <tooltip heading="Rename">Rename the project item.
    <disabledText>No project loaded.</disabledText>
  </tooltip>
</button>
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

PlugIn Class
PlugIn Members