If you're not able to get SSL/SSH connections working, you can attach a password to an upload form (upload scripts have been discussed to death here. Consult Super Search). This means sending a password in cleartext, but you can mitigate that risk by using a one-time pad.

It works like this: Establish a list of words (randomly, one supposes), and give a copy of the list to the person who'll be updating the website. The upload CGI script gets the other copy. Each time a file gets uploaded, the upload script checks the first word on the list, deleting the word if it matches (or rejecting the upload if it doesn't match). The sender scratches the word of their copy of the list. If some l337 h4x0r sniffs packets and grabs the password, no damage is done: that password is now stale.

Alternatively, the upload script can try to match/remove a word from anywhere in the list. This allows you to split the list into pieces that you can give to different people.

The downside of this scheme is that it you'll need to periodically generate and distribute new lists.