No, other programs like Paintshop Pro or GIMP will work. (You might want to look into the Windows version of GIMP, it's free, and almost as good as Photoshop from what I hear.) To use the sig, save it as a .jpg and go to
http://www.dodstudios.net/uploads/upload.php and upload the image. Then, go to your profile, and put the URL that the upload site gave you in tags. You should be good to go.