how to handle background images in grid cell

I have a need to add an image to an editable grid cell based on some critera. When I add the image it appears but when I edit the cell I no longer get the dirty cell image (red triangle in upper left corner). I have tried to add an image two different ways but only one worked? I there a way to add an image and still obtain the dirty cell marker after the edit? If I am not doing this correctly please advise.

Code:

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  private class CustomGridCellRenderer implements GridCellRenderer<ModelData> {
        @Override
        public Object render(ModelData model,
                            String property,
                            ColumnData config,
                            int rowIndex,
                            int colIndex,
                            ListStore<ModelData> store,
                            Grid<ModelData> grid) {
            Integer baseValue = model.get(property);
           
            Element cell = grid.getView().getCell(rowIndex, colIndex);
           
            if ( cell != null && property != null && property.contains("eqmt") && !property.contains("transcom")){
                cell.getStyle().clearProperty("background");
// this didn't work
                cell.getStyle().setBackgroundImage("url(../images/default/grid/note.gif)");
// this did
//                config.style = "background: transparent no-repeat right top; background-image: url(../images/default/grid/note.gif);";
            }
           
            Integer positiveModifierValue = model.get(property + "-positive-modifier");
            String cellContents;
            if ( positiveModifierValue == null || positiveModifierValue == 0 )
                cellContents = baseValue.toString();
            else
                cellContents = "<span class=\"modified-table-cell\">"
                        + baseValue + " ( +" + positiveModifierValue
                        + " )</span>";
            if ( "0".equals(cellContents) )
                cellContents = "";
            return cellContents;
        }
    };

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