I do the same as ChristS and vocal coach, but what I also like to do in my how to articles, is enhance the photo by adding either callouts or numbered steps, so that the learner cannot only follow the visual with each step but to also point out specific areas of the photo or image which are directly related to the step. For example, if there is a picture of the back of a computer, and I was referring to the video input, I would then put a box or circular outline around the port in the image, preferably in a bright orange, yellow or red color, so that it's easy for the learner to locate what I am trying to point out.
For enhancing screen images or photos I love to use a software package by TechSmith called Snag-It. It is an excellent screen capture tool and photo editor that allows you to add captions, lines, arrows and other figures to enhance your photos. Many times I use it instead of Photoshop just because it's a quicker and easier way to enhance images.
Hope this helps.