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<comboBox 
 id="myComboBox" 
 label="My ComboBox:"
 getItemSupertip="CallbackCBGetItemSupertip" 
 imageMso="PictureEffectsGlowGallery" 
 onChange="MyComboBoxCallbackOnChange" 
 sizeString= "WWWWWWWWWWWWW">
</comboBox>

Sub CallbackCBGetItemSupertip(control As IRibbonControl, _
                              index As Integer, _
                              ByRef supertip)
    Select Case control.ID
        Case "myComboBox"
            supertip = Nz(DLookup("txtSubName", "tblPictures", _
                                  "txtPicName='" & _
                                  Format(index, "00") & "'"), " ")
    End Select
End Sub