So, I've been messing around with pixel art in general recently and decided to join a community to try and improve my skills. I did do some in the past but very infrequently (e.g. those old Pokémon fusion thingies that used gen 4 sprites) and also I draw a bit on the side, but would like to focus more on pixel art as I feel I get more invested in that.
I've done a couple of recent examples to show what I can do right now but it's mostly just trying to "draw over" existing stuff:

A few small sprites that went unused for a small college project in 2017. I believe those were made in Piskel (the rest was in Aseprite).

Draw over of
this duck because it's sort of a meme/joke between me and my friends.

Draw over of the mana symbols in Magic.

WIP (basic outline) of a draw over of
Longtusk Cub, trying to mimic a game-like sprite.
I mainly think I should improve color choices (I have zero idea, altho console palettes could be a good start) and the size of the sprites. So far I haven't gone past 32x32 and I don't know how to exactly choose that (or make bigger sprites, for that matter). Thanks in advance for any critique or tip!
PS: I uploaded everything to Imgur - the FAQ link for the Image Upload Guide was broken so I'm not sure what is best.