The answer to this is not exactly straight forward. If you just want to get images efficiently on a background thread then I suggest you use EGOImageLoader which can be found with code examples here. It's an easy to use library that will download images from a URL to a UIImageView.
The part where you get images from social networks like Flicker or Instagram is not as simple. You will have to look into each companies API's. It will require a basic understanding of HTTP requests. You will have to use an OAuth authentication point which brings the user from your app to the other website for them to sign in and then sends them back to your app with a token to use in all further HTTP requests. A good library for easily sending HTTP requests is AFNetworking (found here with code examples). After getting the token you will need to make API calls using it to get the images you need.
As I assume you've found during your research there are almost no native iOS API's and even the ones that exist are only for posting data, not for getting nearly as deep as I assume the OP is looking to go.
but i think this code will download only specific image from URL
and what i want to download pictures in my website i will have it in my app
and when ever i have new image in my website it will be download to my app
Mr ArtOfWarfare
i will try to Build iOS Flickr App but are you sure that it will work because
i find it hard little bit