the way I made the circles was by using the paint brush tool - I set the brush size to the side of the sprite (250 or 500), put the mouse in the middle and clicked. Then set alpha to 0, made the brush smaller (about 246 and 494 I think), and clicked in the middle again.
You could make a 1 pixel circle by reducing the brush by only 2 pixels (leaving 1 pixel on each side), but I thought the circles didn't look as solid as the grew larger... for the 500 pixel circle I reduced the opacity as it grew to see how that would look...