Instructions
Click the "Contact us" tab in the navigation bar of the website where you first found the image. Depending on the website where you found the image, if it is owned by the photographer, there should an email address or phone number where you contact him. If the website is a public gallery like Flickr, just underneath the image will be the name of the photographer or agency that posted the image.
Explain your interest in the image. If you are using it for commercial purposes, then you will need to negotiate a "License" agreement spelling out how the image will be used and for how long. If you are using the image for personal use in your home, then you may be able to purchase a personal use license for less money. If you negotiate a personal use license, you must adhere to those terms. Obtaining the picture under a personal use license and then using the image for commercial purposes may wind you up in court.
Go back to the website after you have negotiated and paid for the license. Click on the image and follow the steps to download the image file to your home computer. Save the image as JPEG at the highest quality if you plan on printing the image in the future.