Changing child's coordinates when zooming uiscrollview

Changing child's coordinates when zooming uiscrollview

Well, I have a scrollview in which I put the image principal MainIMG.
I also put 2 secondary images IMG1 and IMG2.
On the scrollview, i can zoom the principal Image (MainIMG), but not the
secondary images (IMG1 and IMG2).
My problem is : i don't know how to make the secondary images retain their
positions relative to the MainIMG when i zooms.
for example if IMG1 at the center of MainIMG when zoomScale = 1, it shall
be as when zoomScale = 2.
thanks a lot for help