It will spawn on the Spawner.X, Spawner.Y automatically on creating the action. Imagepoint can be left as '0' default for the origin, central point of the Spawner. The act of selecting the Bubble object to spawn on the Spawner will automatically let it know that it needs to spawn it on the Spawner position, without having to set any co-ord position.
Also to clarify, the field 'imagepoint' does not expect a co-ord position, it is the number or name of the imagepoint you want to spawn at. The imagepoint is a position on the Spawner object itself, the origin (0) is there by default but you can place others that increment 1,2,3 etc and can be named. Choosing this allows you to offset the spawning location on a particular object i.e. if you wanted to spawn bubbles on the very top corner of the Spawner object you could do this by creating an imagepoint(1) and referencing it.