About those 1st 2 art. You should practice a little bit of shadows + figure because the 1st is a whale that 2nd is for me unknow. About 3rd art it's quite nice try to do a bigger Version with more details. The 4th art is pretty like wood and rock to, but the last art you should give him some shadows and light shade.
But about all those art I can say only 1 thing that you're doing pretty nice as New guest in Pixel art (I'm like you so yea)
Oh and do a PNG on Backgrounds because on Index it's hard to change colors for editors who want to change a little bit of those sprites ;3
