The planet doesn't look round for two reasons. One is that the rings aren't circular in the picture, because they're slightly oblique to the viewpoint, but you

The planet doesn't look round for two reasons. One is that the rings aren't circular in the picture, because they're slightly oblique to the viewpoint, but your brain expects to see round circles, so it compensates by distorting your perception of the planet relative to the rings. Two is the planet actually isn't round, because of its equatorial bulge, which in this image is slightly more in the up-down direction than left-right, because of the obliquity of the viewpoint. This effect is in the same direction as the rings perception effect and stacks with it.