Call to Undefined Function: wp_constrain_dimensions() When Uploading Images or image_downsize() in Gallery/Media Library in WordPress 2.5

When using WordPress 2.5 Flash based image uploader to upload pictures to the web server, or when clicking on the Media Library or Gallery tab in the WordPress Flash Image Uplaoder, the following PHP error may occurs no matter what web browser user is using, including Internet Explorer and Firefox.

Fatal error: Call to undefined function: wp_constrain_dimensions() in /home/wordpress/public_html/wp-admin/includes/image.php on line 173

Fatal error: Call to undefined function image_downsize() in /home/wordpress/public_html/wp-includes/post.php on line 2326

The issues happen due to corrupted WordPress files. The corruption may happens during the process of files uploading or transferring to the server, or when overwriting or replacing the old version of WordPress files, or caused by a corrupted or invalid upgrading process.

The resolution to the fatal errors is to re-uploading and replacing all the old WordPress files on the web server with the latest WordPress files again. To ensure cleanliness of the upgrade, it’s recommended to delete the following 2 directories before upgrading:

/wp-admin/
/wp-includes/

Then upload and replace the whole directories of wp-admin and wp-includes with WordPress 2.5 files or latest WordPress 2.5.1 or other newer version if available (directly upload the 2 directories), together with all files in the WordPress blog root directory, EXCEPT wp-config.php. The problem should be solved by then.