is_image() problem

[eluser]WanWizard[/eluser]
What is $this? And where is this is_image() method defined? And what does it do?

edit:

If you're referring to the method in the upload class, that just checks the mime type of the file, which doesn't say anything about the actual contents.

You can execute a getimagesize() on the file. If it doesn't return FALSE, you're fairly sure it's a valid image.