Excellent job.

Last detail, although i don't think that in this age is a problem, (if it was, it would be on the F.A.Q.) You might want to convert your file to 256 colors format.
This will make sure that pixels in the figure gets associated with a color in the palette. This action is called "indexing"

It's just a tiny detail, like when a waiter pours the content of a bottle in customer's glass, it is something most developer can do personally in a fingersnap, but is the last step to do, in order to provide a sprite that's really "ready to be coded in any project"

Now, this axe is rather simple, but in more complex design, you might want to make sure the computer understands what belongs to the blade, what belongs to the handle, and what belongs to the coating. (the most common example for this might be the white in the eyes, if a character is wearing a white T-shirt, you must specify that the white of the eyes and the white in the T-shirt are 2 different colors, so when someone decides to turn the T-shirt blue, the white of the eyes doesn't turn blue along.)

With 4 cels might be a quick job, but if a sprite has 50 or more cels, and the things sharing the same colors are more (A blade and the shoes, a holster and the belt, etc. etc.) It is a good idea to make sure that changing the color of an element doesn't change another element that's not supposed to change.

Now, i un-indexed an image made by me to explain the concept. When i ask to the computer to apply the palette, the computer thinks the white in the eyes is part of the jacket, and when i index it and decide i like more a green jacket, the white of the eyes becomes... well, green.
had it be a spritesheet with 400 cels, indexing, it would have required to re-paint the white of the eyes.

I'm sure you'll learn this in very short time.