The way that you have the add to cart setup seems fine to me. It looks like it should add the name of the uploaded image to the cart. Can you confirm that the additional images are getting uploaded to the server? In the add to cart server behavior you can try updating the trigger to be based on 'file upload successful'. This should prevent the add to cart from occurring if there are any problems with the upload.
You can also check on the value that is being inserted into the cart before the add to cart occurs by adding in this code just after line 174:
die("the uploaded file is: " .$WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"]);