I've been customizing ringtones on my A930 for weeks. I use basic editing software, save as .wav and convert to .mid.mp3
You're not sending it as a pix message per se, but that is how the phone interprets it as incoming when you receive it.
Like paulcman says, bite the bullet and shell out the .25 cents per; that's 12 of your own ringtones, compared to one for $2.99 that you'd get off of Get It Now; the biggest difference (besides the price) being you can use any song, sound, etc. etc. you want...not just what they have available.