property Items.CellImages ([Item as Variant], [ColIndex as Variant]) as Variant
Specifies an additional list of icons shown in the cell.

TypeDescription
Item as Variant A long expression that indicates the item's handle.
ColIndex as Variant A long expression that indicates the column's index, a string expression that indicates the column's caption or the column's key. 
Variant A string expression that indicates the list of icons shown in the cell.
The CellImages property assigns multiple icons to a cell. The CellImage property assign a single icon to the cell. Instead if multiple icons need to be assigned to a single cell you have to use the CellImages property. The CellImages property takes a list of additional icons and display them in the cell. The list is separated by ','  and should contain numbers that represent indexes to Images list collection. 

The following sample assigns the first and the third icon to the cell:

With ExplorerTree1.Groups(0).Items
        .CellImages(.ItemByIndex(0), 1) = "1,3"
End With