Even though I do post a lot of pictures, it is amazing to me how often I forget to hit the actual upload button.
Once you attach your pictures, you have to widen out the screen to actually see the upload button to hit.
These are some screen shots I took before, showing the default layout and then the stretched one. You browser/screen will probably look different but it's the shape that matters.
If you get the "your upload has failed" message, that usually means your file size is too large. I go to this site, follow their defaults and prompts, and download from there.
https://www.reduceimages.com/