I've made some pretty cool shock waves before:
1)Make a sprite
2)In the picture editor, make the size of the drawing about the size of the screen, but square. (so like 500x500 or something, I guess it depends, though its not that important.)
3)Draw a huge white circle over the whole thing.
4)Get out the brush tool and type in the size thing about 20 less than the size of your entire sprite, set the hardness to 0 (or something small), and choose the colour opacity 0.
5)Once you've set up the brush draw a dot in the middle (you can see the mouse coordinates at the bottom)
6)Press 5 on the numpad to centre the hotspot.
7)Sprite is done, apply a magnify effect
Make some event create it, and add to this, set sprite width to 1 set sprite height to 1
9)Add event: always -> set sprite height to .height+1000*timedelta
set sprite width to .height
set opacity to .opacity-100*timedelta
10) add event: sprite.opacity < 1 -> destroy sprite
11)Fiddle about with the constants until it behaves how you want
I can make an example later if you want.