To make a signature, you have a couple options:
1. Create the image yourself, using an image editing program like Photoshop, Fireworks, Illustrator, etc. (You can even use MS Paint if you are so inclined)
2. Use a banner creation tool from a website like www.bannerfans.com. I prefer banners over actual signature makers because they are not as limited as far as I can tell. Just make sure that you can download it after you are finished in a .jpeg or similar format.
I would reccommend making your own if you can, that way your only limitation is your ability to manipulate the software you are using. If anyone wants help using Photoshop, PM me.
Note: I would use something close to 500 px(w) by 100 px(h) if you make your own. This reduces the overall file size but still gives you enough space to showcase your creativity.
Kim-Lan already has a fantastic tutorial on how to upload/embed pictures: