Well, I'll just focus on the shark for now.
The main thing here is to shade according to the actual form of the object, rather than on the edges, as it flattens the form into a sticker of sorts. Another thing is actually portraying the detail of the object - here it's hard to see what the object really is, though it has the silhouette of a shark. For example, the bump at the middle of the belly? Is it a fin? A blade cutting into the poor animal? Something else entirely? Using shading and 'lines' to show what you want to portray is helpful. (Humans naturally understand lines- you don't need to teach people to see that a line represents the edge of something or whatever.)
I'll try to explain with this picture...

(sorry it's not pixelart, I'd spend more time refining that than explaining what I was trying to say...)
What to do regarding this? I say study basic drawing on the side. You don't need to drop pixeling entirely for the duration, but studying on the side certainly will improve your abilities as a pixel artist.
