The safari browser that's already installed on your iPhone can view most types of images on the internet.

A new feature introduced with the latest version of the safari browser on your iPhone is the ability to save images to your 'Photo' Album. Just hold your finger on the image for a few seconds and you'll get the option to 'save image'.

NOTE: The iPhone CANNOT currently view Macromedia Flash or Shockwave. So you have to be sure that the porn sites you use provides content in other formats.