If you are having trouble assigning images to all your products, you can generate a Products Without Images file that shows you all the products that have not been assigned an image.
From the Control Panel home page click on Administration > File Upload/Download
Next click on the Download Files tab, select Products Without Images from the first drop-down menu and then click on Download Data File
Save this file to a location on your PC.
This will allow you to easily review which products you have not upload images for yet/correctly.