When I get that type of error message (this might not work for everyone) just move error msg from uploader to the side & click within an empty photo box in listing. Usually even though I received the error failed to upload, doing this little work around my pictures show up!
Getting duplicate photos supports what I'm saying above. Because the 1st time the photos did actually load in spite of error msg. Just click in the pic box of your listing & see if populated. If not, click pic box again to see if pics were uploaded & then you can select. Hope that makes sense.
Re: only uploading 16 pics at a time, I use to have that problem. (Ditto CPU usage issue.) Then I started reducing the size of my pics from being sometimes 2 or 3 mb down to 1 or lower. Just make sure pics still meet eBay criteria for min size of pic.