Since ive seen a few people asking this here is a simple guide.
First you login - naturally 
Next click on the My Profile button near the top as shown below..
The next screen you will see will have many different options, your concern for signatures is the signature section, you can see that the wite text box to its right is where you post your signature detals, from it you can see you have to use html code as shown below.
URL? - ok to have a signature you need to have your actual signature hosted online, a popular one at the moment as recommended by several people is http://photobucket.com/. Once you have it hosted you will be given the url, which is enterered after img tag in quotes for src.
Now thats done, just click on Save Changes, and your signature has now been applied.
You can always modify further by using hyperlinked signatures or various text etc..
Hope this helps people....Im sure in time once mxo is on the main station site this will become a must
Anyone wishing to add a guide on uploading the image with photobucket please do 
