UIImageView *view ;
. . .
view.frame = rect ;
view.image = image ;

[iview1 removeFromSuperview] ;
iview1.hidden = NO ;
iview1.image = 0 ;


[UIView animateWithDuration:5 animations:
^{
    scrollview.contentOffset = offset ;
    iview2.frame = rect ;
 } completion:^(BOOL finished) {
    iview2.image = image ;
    [scrollview addSubview:iview2] ;
    iview2.alpha = 1.0 ;
 }];

UIImageView* view;
    ...
    view.alpha = 0.0f;
    UIView animateWithDuration:0.2 animations:^{
        view.frame = rect;
        view.image = image;
    }completion:^(BOOL finished){
        view.alpha = 1.0f;
    }

[iview1 removeFromSuperview] ;
iview1.hidden = YES;
iview1.image = nil ;

[scrollview addSubview:iview2];
iview2.alpha = 0;
iview2.image = image ; //you could make this assignment in the completion block as well

[UIView animateWithDuration:5 animations:
^{
    scrollview.contentOffset = offset ;
    iview2.frame = rect ;
 } completion:^(BOOL finished) {

    iview2.alpha = 1.0 ;
 }];

[iview1 removeFromSuperview];

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(NSEC_PER_SEC * 0.001)), dispatch_get_main_queue(), ^{
    [UIView animateWithDuration:5 animations:^{
        scrollview.contentOffset = offset;
    } completion:^(BOOL finished) {
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(NSEC_PER_SEC * 0.001)), dispatch_get_main_queue(), ^{
            iview2.frame = rect;
            iview2.image = image;
            [scrollview addSubview:iview2];
        });
    }];
});