If you have to use an image, check out add_column method
http://phpgrid.com/documentation/add_column/

In the formatter function, you can return anything including image. Make sure to use only SINGLE QUOTE.
e.g.
$col_formatter = <<function(cellvalue, options, rowObject){
return '<img src=http://icons.iconarchive.com/icons/webiconset/mobile/32/maps-icon.png>';
}
COLFORMATTER;