I am enjoying the enthusiasm of your posts! I am reasonably computer savvy but it took me quite a long time to figure out posting pictures here.
For me the issue was I was using an old school digital camera, I have to reduce the jpg files no matter what the size. I use this site, they have some contribution level options but I use the free ones, and just select their suggestions. Remember to not only reduce, but to download. That will also make the files easy to find when uploading here.
https://www.reduceimages.com/
My browser protections were also preventing the pop up for the attach files, I've learned to do a ctrl click for a fresh window. But you seem to have gotten that far.
Have you tried just one picture at a time? And different pictures in case it is some sort of corrupted file??