Downloading images from Url is very common task in iOS apps. It is however important to limit the amount of threads being used as otherwise it gets pretty heavy on the CPU and whole device can be easily clogged up.
There is a nice example from Redth on github to use, however I had problems running it as it failed to add operation to NSOperationQueue for me.
So I rewrote it to use Task Parallels from .NET 4 and it works like a charm.
Here it is on GitHub, enjoy and happy coding!
https://github.com/sichy/UrlImageStore