Bet this is the problem:
"CURRENT IMAGE LIMITS
We currently allow 8 images to be embedded in a post. Embedded images must fit into 1280 x 1280 box, and be less than 2.5 MB in file size"
So just decreasing image dimensions (1280x1280) is only half the issue, you're also going to need to take that file and compress it to 2.5mb or less.
It works because when you do a screen capture, whatever screen capture program you're using is doing this process for you based on your monitor dimensions (I'm guessing you run 2560x1440), however the screen capture program is getting you file sizes of less than 2.5mb, so when you upload them here, it will take 'em.
If you're trying to just upload an edited copy above 2.5mb, the server times out because it's not worth it for an image host to use that much bandwidth when viewers don't need/want your original giant file, they only need a smaller file to fill most of the monitor).
Facebook and other sites will do the compression work for you, that's why it will work on those and not on here. Here you have to do your own file downsizes and compression (unless we all start chipping in big time $$$ for bandwidth).
Hope this helps, and again, it's me guessing at the issue...could be something else.