Is the class this method resides in "Picture"? If so, you can just use in the main:
pictObj.changeWhole(50.0); for example. That should be the correct way to call it.
That doesn't mean you won't get other errors, but the method itself appears to be written to prevent errors as much as possible. The only one that should be possible to throw an error is pixelArray.length, which will only occur if this.getPixels() returns void or null.
That assumes you are not referring to a compilation error though.