Jupiter with Io and Io's shadow.
The non-circular shape of the Io is due to the dark poles of this moon, which tend to become invisible after waveletting, causing the moon to appear elliptical. Compare the shape to Io's shadow, which is clearly round (or in this case, elliptical vertically due to limb foreshortening because it is projected onto "spherical" Jupiter).

Images with Io in transit clearly show this polar darkening but allow the whole moon to be visible.