This might not work for you & you might have to do more than once to get all your photos. If you get that message, move window aside & click an empty photo box within your listing.
In my case the photos have been uploaded, the error msg is just a glitch. Today this happened & 1st just 1 photo uploaded with my method. Then I repeated selecting rest of photos, rec'd error as you show & I clicked another empty box in listing & YEAH rest of my photos are uploaded.
Hope this helps.