I understand that you want to keep your picker inside the circle image. To do that just simply grab center point (p1) of your circle image and center point (p2) of currnet position of the picker. Calculate distans between both:
float distance = sqrt((p2.x-p1.x)*(p2.x-p1.x)+(p2.y-p1.y)*(p2.y-p1.y));
And if the distance is more that circle radius stop move your picker:
if(distance <= radius)
return YES;// Let the picker move, it's inside the circle image
else
return NO; // Picker outside the circle image
Hope this is what you are about.