Filename.png: File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.
It’s a random upload error. We aren’t sure why it’s happening. In the meantime what I’d suggest is uploading it to a different image hosting site (such as photobucket or deviantart) and posting the pics from there by opening the image in a new tab and then pasting the link into the Insert/ edit image box in the top bar of the reply box. That always works.