This Helper I use to store and resize Images (external or not) – it uses GD so make sure to have it, of course this is just an example, so you must change the structure of paths/URLs to your needs.
In a recent Situation i needed to display the attachments of an Wordpress Post, the Images, in a click-through-like manner. I also decided to display the current number in relation to the full number of Images you are currently looking at, like a simple “Picture 5 of 7″. Since there is no build in Option for that I had to build my own.