class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
I assume that your php.ini is correct if Wordpress works.
upload_tmp_dir can be empty ("no values" in a phpinfo()), in which case PHP will use the system default temp directory.
However, your dump indicates that the error code is 6, which is UPLOAD_ERR_NO_TMP_DIR: "Value: 6; Missing a temporary folder. Introduced in PHP 4.3.10 and PHP 5.0.3"
So there is definitively a problem with your temp folder. Try to set it explicitly in your php.ini.Reply
upload_tmp_dir can be empty ("no values" in a phpinfo()), in which case PHP will use the system default temp directory.
However, your dump indicates that the error code is 6, which is UPLOAD_ERR_NO_TMP_DIR: "Value: 6; Missing a temporary folder. Introduced in PHP 4.3.10 and PHP 5.0.3"
So there is definitively a problem with your temp folder. Try to set it explicitly in your php.ini.