I like this, looks impressive... there were some small things however. I'm no expert, but I find the whole straight black around the eyes a little bit weird... maybe shading that part a little or something, looks kinda meh considering the rest of the picture is really well done... also I noted that you went 2 pixels in lines on the top part of the face, check that out too.
About the file extension... .gif is fine, although I prefer .png. To resize a picture, there are many ways to do it... just go based by 1x, 2x, 3x etc... that's the way to make it look perfect. In a program that resizes/expands images, if the current image is 200x200, just set it up to 400x400 so that way it becomes 2x in size, and doesn't look blurry... of course, just mark it as resize, not resample.
Now, for my sprites, I do a more primitive action lol. I just expand the area of the image, and zoom it in ms. paint to 2x, I take an PRINT SCREEN action which you can do by hitting the PRINT SCREEN button on your keyboard, and then paste it in the same image... after this you just highlight the 2x sprite and CTRL + C (copy it) then CTRL + Z twice, to undo both highlighting and pasting the entire screen capture, and end it by CTRL + V (paste). Now you have an easy fast way of copying it right away, just put it next to it or something.
On this site, users can zoom the sprite by just clicking on it, I don't know how to restore the zooming to 1x though, maybe someone from the staff can help answer this question.