@ King Alfonzo :
Thanks for your advices, really appreciate it. It really helps toupgrade sprites.
A question a little bit odd, but how do someone "grebble" in a good way? Any time I try, it seems that they pop to much in my face and I only see pixel after that...
It's something of an acquired skill that requires patience.
For an example, I'll have a random stab at that hound-combo sprite you made. This isn't what I think you should do with it - just an example of how to greeble. First, apply a dark colour to the region you're going to greeble.
Then draw random lines and shapes with a 2 pixel paintbrush tool in a midrange colour to produce pipes or maybe substructure - you don't know at this point. Just keep the shapes and ideas in your head. For inspiration, look at HELMUT's sprite junkyard, Tiangdong ships or D-class ships from the game.
I need to stress this - use a paintbrush, not a pencil. If you use a pencil, it looks very pixelly - the use of the paintbrush helps reduce this. Further, aim to use the eye-dropper tool here instead of making up a new colour - just grab a colour from somewhere else on the sprite, and use that one. Helps keep the colours uniform.
Then highlight the shapes you just drew, thinking about what each of these shapes are, and what they could be. Struts? Pipes? Underlying machinery?
Then apply a dark colour, to see if you can make these shapes you're thinking of pop out, or make entirely new shapes using the dark colour.
Yes, the greeble may be ruined - at several points you will ruin the greeble. But this is a process of refinement. Just apply dark, midrange and light colours over and over and over again, adding in details you might like, then drawing over them when a new idea comes. In this case, I think "No, wait, maybe there should be an understructure, like a ship structure underneath the armour layer..."
...then I change my mind a bit, going through with a darker colour and make some plates and gaps...
I don't really like that, but I do kind of like making the engine more exposed...
...and then I think that the front can be encased in substructure (like before) while the back is more exposed engine...
I just refine over and over, until I find a sort of structure I like. I then do the standard touch-ups as if I was doing a normal sprite.

It's still not perfect, but you get the idea. Just keep going, over and over, backing out to an unzoomed view and wonder if it looks alright, and do not be afraid to start over - just make sure you paint over what you've done previously, as sometimes greebles can just magically form when you do that. Hope this helps.