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

Sub CallbackCBGetItemScreentip(control As IRibbonControl, _
                               index As Integer, _
                               ByRef screentip)
    Select Case control.ID
        Case "myComboBox"
            screentip = DLookup("txtName", "tblPictures", _
                                "txtPicName='" & _
                                Format(index, "00") & "'")
    End Select
End Sub