How to trace from tablets?

How do you trace from tablets or smartphones? Specifically, how do you make an image in the tablet stand still without accidentally moving or zooming it?

Is there an image app on Google Play that allows for locking down the image after you zoomed or moved it?

Hm... i did it from my tablet once by putting a piece of paper over it and then making few light lines in order to get the exact measurements of the figure (we had to compare our own measurements to something related to our major) getting an exact and complete trace would be nearly impossible I think. The only thing that makes it a bit easier is to take a screen shot and then open it in a gallery to avoid pushing buttons while you are trying to draw :)

If you need to do it I would recommend just printing it... I wish I had taken the time to do that - it would have saved me so much... wel...l time :)