This is not a WordPress issue, it is a PHP upload issue. You might get their help by explaining it that way.
Also, do you have a folder under your home directory called /html/tmp? If not, create it.
And if that doesn't work, do this:
Open a plain text editor and create a file called phpinfo.php. Insert the following line:
<? phpinfo(); ?>
Save this file to the root of your Web site and then browse to yourdomain.com/phpinfo.php to test the settings. Find the line for upload_tmp_dir. What does it say, and does that directory exist?