I don't think this is easily doable due to how excel manages images within a workbook.
When you add an image in excel it "copies" the image. If you overwrite the image with a new one excel doesn't care. It has a copy of the original. You could create a macro that dynamically grabs the most current version of the image file and embed into excel. Your tabcmd would have to overwrite the existing image file every time you do an export. The macro would point to the location that you have tabcmd saving the image file and load the new image into the specified cell.
+1 on Aaron's response. Nope.