dhtnmlxGrid with combo column

Hello i have dhtmlx 2.5 pro.
I have a dhtmlxGrid object that have a column of type “combo”.
Before load the xml of the grid i declare de combo options of this way.

“var combo = myGrid.getColumnCombo(combo column);

When the xml of the grid return, its ok, but this grid can add rows dynamically, I need to add a default value.

Other problem is the img of the combo not appear because the path to the img obtains from the grid path.

Thanks in advance.