Caution: *DO NOT* trust $_FILES['userfile']['type'] to verify the uploaded filetype; if you do so your server could be compromised. I'll show you why below:
The manual (if you scroll above) states: $_FILES['userfile']['type'] - The mime type of the file, if the browser provided this information. An example would be "image/gif".
Be reminded that this mime type can easily be faked as PHP doesn't go very far in verifying whether it really is what the end user reported!
So, someone could upload a nasty .php script as an "image/gif" and execute the url to the "image".
My best bet would be for you to check the extension of the file and using exif_imagetype() to check for valid images. Many people have suggested the use of getimagesize() which returns an array if the file is indeed an image and false otherwise, but exif_imagetype() is much faster. (the manual says it so)