I have been struggling with this as well, and I came back again to see if I could get some answers. I got frustrated again because my results just didn't work, so I did a Google search and found a website that is very helpful. Forget all this complicated stuff and just go here... https://anyimage.io/ It takes care of everything for you. This will be so helpful for all of my clients, and I can even do it for them and give them a new link to use on Facebook. Good news for everyone!