It's easier in proper image editing programs, but here are instructions for "mspaint" which is on all Windows computers.
Save image to disk. Open it in mspaint. Click the tool that looks like a dotted rectangle. Draw a box around the bit you want to keep. Go to Edit->Copy.
Go to Image->Attrbutes. Set width and height to 10 pixels. Go to Edit->Paste. Say Yes if it asks "Do you want to resize?". Go to File->Save and save the file as jpg or gif.
Or download this ready-made version:
http://lebutler.lunarpages.com/Signature.JPG
Note the image is still too large to use as a sig here at AHS, maximum size is currently 527x350 pixels while Signature.JPG is 646x217.