Greetings DuoDynamo, make yourself comfortable here.^^
Since you are a fresh graphic artist we will go somewhat easy on you. Our advice will be honest, but do not expect us to sugarcoat our feedback extra for you.
If our criticism is like a hard hit in your belly, please do not think bad of us. We will try to help you as good as we can, but it is required that you listen to us and learn from us, as well the mistakes we did and you will do. Now that we are done with greeting and the warning, now let us go down right to give you criticism.
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First of all please save your work it as a bmp or a png. Saving it as a jpeg or jpg, ruins your work. As it is now it wouldn't be able even to be used in a NES game, because it has now on each pixel a different colour, even if they look the same. The colours needs to be consistent and actually have the same values of which the colours are made of.
One sprite layer and tile can only have 3+1 colours. 3 colours from what you have chosen and 1 for the spritesheet it is on. In a NES game the background colour would be the internal ROM itself. However it is possible to use more then one sprite layer.
For presentation I will show you one of my works.
Now a big applause for the Ultra Master Zest.
Ultra Master Zest is a sprite consistent of two sprite layers, which means he uses 6+2 colours. Truth to be told, he is only using 5+2 colours, because both sprite layers share the same colour. A sprite layer can be of any size, but the greater the sprite layer is, so greater is the chance the sprites and the layer start to flicker.
What is flickering? Flickering is the inability of the NES to display more then 8 sprites in a line at the same time.
The size for a single sprite and its layers are 8x8 in pixels in lenght and height. When the size exceeds 64 pixels in lenght or height on the screen one the very same line, flickering will occur. During flickering the sprites will go invisible at random.
Back, when I didn't know about the colour limit, I took things a bit too soft. As a result I created sprites, which would consist of too many sprite layers, which in turn would make the sprite in practice useless.
This here is one of my older sprites, which I called Armor Man. As you can see he is only consistent of 5 colours, but because he needs so many sprite layers, he will cause flickering with Megaman on the same screen. Two lines have 7 sprites as you can see, by counting the colours on a single 8x8 sprite. If it has 3 colours or less, it only uses one layer. 4 till 6 colours and it uses two layers.
The NES might be able to handle the colour count, but not the sprite count.
Sometimes it is just better to utilise 3+1 on a big sprite layer, while your second palette consists only of one 8x8 sprite.
My Ultra Master Zest is made with NES limitations in mind, therefore he will work in a NES game, if is ever placed in a game.
Tiles are a different story, as they making up your background. They are also used as Pseudo sprites on black screens as you have seen them in various Megaman games as the so much called fortress bosses.
One thing what you can keep in mind is that 24+8 colours are possible to be displayed on the NES at one screen at a time.
4 palettes for the sprites which are 12+4 colours and 4 palettes for the tiles which are also 12+4 colours.
Once you have fixed your sprites, I will comment on them. If you have any questions, please ask.
Edit: Before I forget, use for NES graphics, these colours. These are the colours the NES can and will only be able to use.