I find that I have trouble uploading pics if the page is still downloading. This is particularly evident when the page downloading has many full-size (not thumbnail) pics.
I only wish that posters would upload thumbnails instead of full-size.
It also happened to me just now. I was able to upload two images on a post, but the third image hung up and gave me an error "Something went wrong, contact Administrator" etc.
Might be an issue of running out of space somewhere in the system.