You need to study light and its behaviour to bring substantial realism to a piece. There's a wealth of information on the subject. A book will be your best source of information, but you could probably find decent
resources on the net, also.
A question I ask is, can you shade a sphere believably? Can you bring it close to looking like a photo of, say, a ball? If you cannot, you do not understand 3d form adequately to shade anything (let alone a complex compound form like these creatures) in a believable fashion.
A study of anatomy will also help you design creatures that are believable and have an obvious physical operation to them. The hands, feet, arms, legs, wings, eyes, and heads could all benefit greatly from this.
The mirroring of the middle creature is bothersome to me. It's much more interesting compositionally and feels more natural to render it in a more asymmetric projection.
