In typical 'you never get something for nothing' fashion, the greatest asset of a jpg image is also its greatest downfall. Most people know that a jpg image is a compressed image, which means that it has been compressed using an algorithm that makes the file size smaller. This can be a huge advantage when trying to transfer the image electronically, which is what you're doing every time someone looks at your webpage. Some people know that the method used to create jpg images is what is called a 'lossy' compression algorithm, which means the image you end up isn't quite the same as the image you started out with. What a lot of people don't know is that every time you save a jpg image you decrease the quality of that image, which can be a real problem if you don't pay attention to what you're doing.

JPEG (pronounced "jay-peg") stands for Joint Photographic Experts Group, which is the name of the committee that originally wrote the standard. File extensions for this file type are jpg, jpeg, and jpe.

As a picture is worth a thousand words I'll give a couple of examples. What I did here was to open an image of my logo, which was originally created using Photoshop, and then created a jpg copy of that original file. I then opened that copy using Microsoft Paint, created another copy, and then saved that copy over itself about 20 times without making any changes to it. Here is an extreme enlargement (1200%) of the original jpg image created with Photoshop:

Here is what that image looked like after I performed multiple save actions in Paint, at the same enlargement:

You can see a noticeable degradation of the original image. Now, there are a couple of important points to make here:

1. Any good graphics program, such as Photoshop, will allow you to select the amount of compression to use when saving your image as a jpg. Obviously, the higher the compression the lower your resulting quality will be.
2. The original image, shown at the top, had a file size of 63kb. The bottom image had a file size of 29kb. It is your classic trade off of quality versus file size.
3. The vast majority of the degradation occurred in the first save action by Microsoft Paint, where I do not get to choose the amount of compression to use. However, each subsequent save action further degrades that image as it is compressed just a little bit more.

This is not meant as a critique of Microsoft Paint, I could have achieved the exact same results using a very high compression rate in Photoshop. The reason I used Paint is to make this single point - you can forever change the quality of your original image by simply saving it. For that reason alone you should never, ever save over your original jpg image. Believe me, I know what it's like - you're in a hurry and without even thinking about it you click on 'Save', when all you really wanted to do was take a quick look at the image and then close it without making any changes. Too late, the original image is now gone - forever.

If you haven't already done so, create a workflow where you pull your original images into a safe place, a file location or external drive where you never modify them. Once you have your originals in a safe place you can create a copy of that image for any kind of modifications. That way you will always have the original file, in its original form and quality should you ever want to do anything else with it. The second you click 'Save' when working on your original jpg file you will lose the original forever (unless you have a backup). You can not return this new lower quality image back into its original higher quality source. Once compressed the original information is lost, and every subsequent save action on that jpg image further reduces its original quality.

Additional resources

• Wikipedia has good information on lossy compression here
• Get it from the source - http://www.jpeg.org/

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