Even the CommanderD ones?
As far as the robot head goes, there's not much even pointing to it's status as a robot. It looks more like a megaman energy capsule. Machines are functional, each part has a purpose, and your robot looks more like a storage unit. If a machine has eyes, that means it has a way of receiving visual input, if it doesn't it probably means it is blind or takes aural input. These things go into the design. Because robots can be created and designed by humans, they often share a lot of similarities because of sentimental value and it's what is known to work. To draw something you must understand it. Also, a robot head doesn't necessarily have to look like a head to be understood as a head if you place it upon a body where the head is found. Even if it's just shoulders you add, it could really help this come across as a head and not a <3 mini fridge <3
On a pixel level, you are not economic. You would do well to make better use of each pixel, each color slot. If smooth gradients are found in a palette such as your frog one, it could mean that there is some badass gradated rendering, but a lot of the time it just means unnecessary blur. The digital artist is always fighting blur, at least I am, anyway. In digital painting I find to get nice lines I'll have to sharpen either manually or with a filter, in pixel art I get it by being more economic and having smart pixel placements and a palette with appropriate contrast.
On a general art level, I think what will help your rendering the most will be to consider everything as a separate object, not just THIS IS FROG, but THIS IS A SERIES OF 3D SHAPES. Shade each part of the frog, break it up into spheres cylinders, rectangular prisms etc. It will come as second nature in time, and you will learn how to render objects under a variety of lighting conditions and from any angle. You don't even have to look at that edit you were given, look at your own hand. Look at the way the light falls. It's darker in some places, and lighter in some, because of where the light reaches and can not reach. The light bounces around the room, the light will reflect off smoother surfaces, it all has to do with light. Once you understand light, you will be able to render efficiently. If you do not, you will fail very often.