This help page explains some common reasons for upload problems and how to resolve them:
That said, a tiny fraction of a percent of users have started reporting random upload issues in the past several months; unfortunately, we haven't been able to identify any apparent cause or common factor so far -- in fact, the issue recently cleared up significantly just as mysteriously as it began, all without any changes on our part.
It seems to be a matter of upstream network issues somewhere along the path across the Internet between affected users' ISP connections and our Tucson datacenter, rather than any hardware/software issues on our users' own computers or in our own systems. If possible, you might try rebooting your router (if there's no power switch, just pull the power plug, wait 10 sec., and plug it back in), or retry the upload from another location with a different Internet connection.
If that's not practical or still doesn't work, please email us to arrange an alternate upload method: