If you have already uploaded some images and you want to see which items do not have images, you can view this from the Catalog.
Click on the "All Items" button, that will switch to display "Missing Images". Then click on it again when you want to return to the regular view of "All items":
Another way to get an Excel report of items that have no images, is to go to the Items -> Import/Export tab and click on the "Advanced Export" button and click on the "Include Image Status" box.
Then Export your items to Excel, and a column will be added indicating if the item has an image or not.