Images Downlaod

Hi,

I have an application that the listings have many images.These images are downloaded through a service on the internet. I have some questions about the best way to do some things.

I was thinking when I download an image to save to a folder of APP. So before fetching the image will see internet now exists locally.

The question is how to retrieve the images asynchronously? Ie loading lists him fill all data put a default image and then download the image change in the list.