Here are two tricks when dealing with images on your iPhone.
No matter what screen you're in, or what application is running, you can take a screenshot at anytime without the need to install any other app. You can do this by pressing and holding the "Home" and sleep button at the same time. The screen will flash, a camera-like sound will be dispatched in the speakers and the screenshot will be stored in your Camera Roll. This is a nice little trick especially if you've just reached a new record score in some game your playing. It gives you bragging rights to your friends.
Saving Images From Within Safari
When you're browsing the web and want to save a cool image on a website, you can simply tap and hold on the image until the menu pops up. From there, you can select "Save Image" and you've just downloaded it onto your iPhone. It's located in your Camera Roll. The catch though is that you are downloading the thumbnail-sized image that you're seeing on the webpage. If the image links to a bigger sized image, you need to open up that link and then perform the tap-and-hold operation.