Initially I couldn't upload images to my WordPress site at all because the upload path was wrong. I deleted the upload path listed to return WordPress to it's default upload settings.
After doing this, images could be "uploaded" via the media uploader, browser upload or by being uploaded directly to the uploads year/month folder through cpanel, however they appeared as grey boxes in the library and only the file name on the attachment page.
Since deleting the incorrect path from the upload path option under Settings>Media, the option to change the path has disappeared from the admin interface.
My next action was to manually set the path as wp-content/uploads in the mysql database to see if that would fix the problem, but all uploads still show as grey boxes.
I've deactivated all plugins and replaced my wp-includes and wp-admin folders. The problem persists.