When you are uploading pics on myspace, how do you put one picture on top of another?

I don't think you can.

If you're looking to make a photo collage, I suggest making it with photo software on your computer then uploading the collage to your online hosting site and inserting it in the usual way. I use Microsoft Picture It!, it has a lot of built in borders and funstuff to make your collage look professional.

A link to free software downloads is attached should you need it.

Good luck