When I search Google Images the results screen presents with images about the size of a postage stamp. I can click on the images and it then produces an image which is about the size of a playing card. Is it possible to increase all the images to full screen? The computer is running Windows XP.

Google Images results pages only display found images at a small size (known as a thumbnail size) because that allows a large range of results to be displayed on screen at once. If the full-sized images were displayed on the results page, you would likely only see one image and need to manually scroll or click to the next page. Additionally, given that some of the images are very large in size that would result in some very large downloads, particularly since you are probably not interested in most of the images and may only be looking for one particular image.

The next part of your question relates to why when you click on an image in the Google Images results it does not produce the full size image. When you click the thumbnail this produces a framed window with the top frame having another thumbnailed version of the image together with information about the image, such as the size, location, and copyright notices. The bottom frame has the original webpage that contains the image. The reason that clicking on the thumbnail does not automatically display the larger size image is because the intermediate framed-window setup allows you to see the image in the original context within the original webpage. This is useful if you want to see the source of the image in case the webpage which includes the image has additional information relating to the image. If the link from the thumbnail in the search results went direct to the original full-sized image then there is no way to trace which webpage that image came from, as the image itself does not include that critical information.

As you can see, the intermediate step within the process makes it much easier and shows the relationship between the images and the source web pages clearly. As far as I am aware, there is no way to change this behaviour as this is simply how Google has been designed to function.