I am pretty sure there are tutorials on the internet.
But uh. Do the circle on a new layer while the layer with the pic is visible. Get it? Add a new layer, select that layer, do the elliptical marquee, rasterize it if need be, then hide the layer with the pic and magic wand the area outside the ellipse; finally show the layer with the pic and select that layer, then press the Del key.
But uh. Do the circle on a new layer while the layer with the pic is visible. Get it? Add a new layer, select that layer, do the elliptical marquee, rasterize it if need be, then hide the layer with the pic and magic wand the area outside the ellipse; finally show the layer with the pic and select that layer, then press the Del key.