How to transfer photos from computer to the Iphone camera roll?

The easiest way is to e-mail those pictures to your e-mail account that you can access thru your iPhone mail client.  Even when you have multiple pictures, it is very easy to multi select and attach all those pictures in one go to your e-mail.

Then, go to your iPhone and access your mail.  Now, click the reply ICON and it comes up with many options.  One of the option to save all the images in the attachment to your iPhone.  When you select this option, it saves all the images into one go into your camera roll.

The advantage of putting your images in camera roll are plenty.  I found it very easy/useful to access them thru Photo Streams from your TV, using Apple TV box. When you access your photos this way, you can see your pictures on your high definition TV with many special effects while paying your favorite music in the back ground thru your home theater. Great way to enjoy photos with family and friends!