I add one picture at a time. Once you browse, select and add a picture....you should be able to repeat the process. I don't think you can add more than one pic at once but I was able to upload three pics to an add one at a time.
For instance here in this forum when you ad a pic it goes in a different "slot" each time. You're saying just ad them "on top of each other." You just can't see the file name, right?