Displaying custom ribbon image using callback

I have this ribbon callback to deterimine the image of a button. As shown it's using the inbuild mso images, which works.

Sub GetButtonImage(control As IRibbonControl, ByRef returnedVal)
    If a = b Then
        returnedVal = "Info"  
    Else
        returnedVal = "AnimationOnclick" 
    End If
End Sub

However, instead of using an mso image I want to use a custom image called "MyImage" that I've added to the XML.
I've tested that MyImage works when hard-coded into the XML, but when trying to set returnedVal = "MyImage" no picture shows.

So in this example, if a = b then no image is shown when it should show the custom image.

Sub GetButtonImage(control As IRibbonControl, ByRef returnedVal)
    If a = b Then
        returnedVal = "MyImage"  
    Else
        returnedVal = "AnimationOnclick" 
    End If
End Sub