I want to change the title position of NSButton that has a background image so that i can place the title in the bottom left corner of the button and in the same time overlaps the image
Sounds like you need to override the drawing methods in NSButtonCell (that's what NSButton uses for drawing). I've created subclasses that "fake out" the NSButtonCell implementation for simple adjustments. To do this, create a subclass of NSButtonCell and set it up as the cell for your NSButton. Next override the drawing methods of NSButtonCell (there are multiple depending on what you're trying to do), adjust your target frame and then call the superclass (NSButtonCell) implementation.
Depending on how much you need to do though, you may just be writing your own drawing methods. This gives you the most control.