What are the things need to take care while designing app for 2 different resolutions ?

It all depends on your image:

• If your Image can be stretched, UIImageView will do all the work.
• If only a part of you image should be stretched you should use this:
• code - imageView.image = [imageView.image resizableImageWithCapInsets:UIEdgeInsetsMake(top, left, bottom, right)];
• If your image can't be stretched you should then do different images for the phones and change them in runtime.