Motorola RAZR V3m Ringtone problem

Ok, I am new to cell phones and I am having trouble with ringtones. I created my own ringtones from the internet and sent them to my phone. I follow the directions and save them to "My sounds"....but when I go into main menu to set one of them as a ringtone, it does not allow me. None of the ringtones that I created are allowed to be set as ringtones....however, just playing around with the settings etc. one of the created ringtones magically appears as a set ringtone. Then out of the blue it will ring the previous ringtone! It must be possible to set the ones I create but how? I need help from you folks.

How long are the ringtones you created? Sometimes if they are too long, the phone will not allow you to apply them as a ringtone.

Also, were they created as mp3's?