set the image property to ScaleToFit.
then
myImage.frame = CGRectMake(160,240,myImage.frame.size.width*mySlid er.value,myImage.frame.size.height*mySlider.value) ;
slider value maybe u can adjust so that it is 1 in the middle. so 1 is the initial size. minimum maybe 0.5 and max is 1.5.
try that.
(or what ashishjraval posted is good too)