The easiest way to copyright images is to simply create them in some medium that is visible or tangible in a real way. Once you do this, regardless of whether the images are digital media that exist only in a computer or illustrations created on paper, the images are protected by copyright law. You may want to further protect your work, however, by registering your copyright with a government agency or private company to ensure you can prove when you created your work. While you can copyright images simply by creating them, you should keep in mind that you cannot copyright ideas that you have or images of characters already under copyright protection by someone else.
In general, the simplest and most direct way to copyright images is to create them. Regardless of how you create the images, you establish a copyright and own the images simply through the act of creating them. This means that if you use a computer program to create an illustration, you own a copyright on that illustration; if you use paint and create an image on a canvas, you have a copyright on that image as well. Although you copyright images the moment you create them, you may want to register your copyright for greater protection.
Some countries, such as the US, have government offices that can register a copyright for businesses or private individuals. Other countries have private companies that handle copyright registration. The process is fairly similar in either situation: the images are submitted with the proper forms and registration fees, at which time the copyright is registered. For legal purposes, you copyright images when you create them, but once you register the copyright it is much easier to legally prove when you established that copyright if necessary.
You should also know that you cannot copyright ideas, nor can you copyright images of someone else’s work. Any photographs you take of a painting someone else created may be your own photographs, but the copyright protection you have for that photograph would be restricted by the copyright on the painting featured in your photograph. Similarly, you cannot copyright images you create of characters owned by someone else. Since the characters are owned by another person, you could only use such images with permission by the owner, though any use that does not reward you financially is often unlikely to cause legal action against you.