It looks like you still have some pixels that have really really close values. Again, that's the problem when randomly choosing pixels while working. What I suggest is that you choose your palette before starting to work.
Once you're done and think you have a good palette, make sure to draw it on your piece, with 2x2 pixels for each color for example. Then, while working on your piece, only pick the colors FROM your palette that is on your piece.
 

Now that's 11 colors + Transparency = 12

By the way, not completely sure where GraphicsGale got it's numbers, but if you'll look at the image closely I used three colors with four shades per color, plus the black background ( 4*3+1=13 ).

@Mhyre: I took your advice and put it on a black background and fixed the missed pixels. It actually does look better.

You don't really have to have GIFs for pixel art with a transparent background. I recommend using paint.NET (free) to set transparency. Simply open your image with it, use magic select (magic wand) then select the pixels that should be transparent (holding CTRL for multiple selects) then press DEL.
Save as PNG > Auto-Detect for bit depth, and you should be good to go!
 

Thanks Mhyre, I was going for something to use in a shooter and, I've come to realize just how difficult it can be to get size right when you're zoomed in enough to see individual pixels.

As for the number of colors, that's what graphics gale counted, which included the layers I used to create the shades, as well as black, white, and transparent.

The bright grey pixels as well as the two black ones next to the cockpit were mistakes, they weren't supposed to be in the finished product.

I did look at the FAQ, and, while it may be a little dfficult. I'll have to transition to GIFs.

Thanks for the comment, hopefully I'll be able to correct everything in my next image.

 Hi and welcome to Pixel Joint!
First, this is incredibly small. Even though this is your first pixel art, 25 colors is way too much for something so small.

Let's have a look at the blue part (which is the cockpit if I'm right). It uses too many blue pixels that are just slightly different, which is useless. For something so small, the human eye can not really distinguish all the different blue pixels.

Next, looking closer and closer, I see bright gray pixels. These are also useless, considering you're using a white background! Backgrounds are not essential to a piece like that, but for such tiny details, a black background or even no background at all (transparency) would be better.

If there's one page you should look at, it's probably the FAQ. It provides all informations you need to properly use this site and best display what you made.
Hope this helped!