I need some feedback for my spritesheet

I decided today that I was going to make a spritesheet for the spriters-resource, but it got rejected only seconds after I put it up for reviewing. This was expected as I am new to spriting. What I really want is for some feedback because I only saw a couple of things wrong. Also just to add I run on a very old mac so I don't use very good software, actually I use the website Piskel for editing, so not very good.

Don't be afraid to speak your mind, don't worry I won't take it harshly or anything.