This knitted nautilus looks great. I like knitting and recently i knit a scarf for myself. What about Aseprite - may be it's not adopted for linux os. There is another one free pixel art software - grafx2 http://code.google.com/p/grafx2/
Great stuff in retro style, but powerfull and it works with all os.
Aseprite actually looks really good (I'm on Linux, though, so I had to compile it). It seems to be somewhat CPU hungry, though.
Here is the knitted piece I was basing this on:
https://lh3.googleusercontent.com/-shwxkN56uoE/TLzddjUsvHI/AAAAAAAAAWA/BuV9qT-Y978/s288/IMG_0522.JPG
Technically it is a prehistoric nautiloid and not really a nautilus (as in chambered nautilus), though.
@philippejugnet: No matter that it looks not naturally, especially if to read the cobrajs comment: "a knitted nautilus". It is not my assessment, if I do not like pixel art. Congratulations on your debut, cobrajs. I am glad that you are using is not photoshop. Try to use Aseprite aseprite.googlecode.com/files/aseprite-0.9.1-beta-win32.zip It is more suited to pixel art.
@Asterix: You look like fisherman right now LOL
this could have lots of improvement in textures, shapes. You could also look for references like:
good luck bro I liked the idea of a blue nautilus.
I love nautilus, but the tentacles and the shell are very strange. 
I think it should be an "ammonite", isn`t it?
I did get it working on Linux, just not with the link you gave me.
Grafx2 is my other favorite tool. Runs on everything that I have to run it on. Another good one for Linux is mtPaint, but it is kind of wierd (I never got used to it).