This will draw the outline of a circle to this Image.

.drawCircle( x, y, diameter, isCentred = false )

Draws the outline of a circle, on this Image, at the location given.

'diameter' is used for both the width and height of the circle.

// Draw lots of circles to an image,
buff = new Image( getScreenWidth(), getScreenHeight() )
10.times() do |i|
    buff.drawCircle( 200, 180, 20 + 20*i, true )

// then draw the image to the screen
onEachFrame() do
    drawImage( buff, 0, 0 )

.drawCircle( x, y, diameter, xAlign, yAlign )

The same 'drawCircle' function, but with x and y alignment options. See the article on How Alignment Works for details.