You can't just simply upload an mp3 to YouTube, it must also come with a video. But if you are using Linux, there is just a simply command that will turn an mp3 and an image into a 'video' ready to upload to YouTube:

ffmpeg -i your.mp3 -i picture%d.jpg out.avi

Obviously, you need to have ffmpeg installed for this, so if you don't:

sudo apt-get install ffmpeg

Not using Linux? There is a simple way for you to do this too. Check out mp32youtube.