JPG's need to be about 1.4Mb or smaller, I usually set the pixels to about 2400 in the larger dimension.
I have never been able to load photos that large and that is way above the limit stated in the pop-up, which is 585.9 KB. May depend on how you are posting them, I use the thumbnails, and this is the limit I use.
I know we're all just trying to help. Here's a post from about two weeks ago where I mentioned the file sizes. It looks like 2000 KB is about the limit on .JPG's.
https://www.fordbarn.com/forum/showpost.php?p=1984415&postcount=9
full thread: https://www.fordbarn.com/forum/showthread.php?t=294363