Have you also tried your example code? Because it doesn't work the way you posted it. :)
You left some important parameters in watermarkText method and in other places.
Here is a working example code: http://pastebin.com/30GTj5Sf
BUT it doesn't mean the class works well: e.g. in my example it put watermark text in the top right corner, BUT bottom left was given.
I would also recommend better error handling: for example no exception is thrown (or no other way to detect errors) if no existing image files were given (watermark or other kind of images), we just get an ugly PHP warning message, and it doesn't even check if given parameters are of "valid" types (for example if a PHP function doesn't accept a string as parameter, we shouldn't pass a string to it).