If you’ve got a digital camera, you’ve probably noticed that it saves files in JPG format. This format is the most common type used in digital photography because it is designed for compressing images. Because of this ability to compress the image file, when you are working with JPGs in your imaging software, it helps to understand what’s happens when you save your files.
Image files can be enormous, so over time image compression schemes were developed to help keep file sizes down. Small file sizes are important if you will be using an image on a Web site or sending it in an e-mail. JPG files use a type of compression termed "lossy" which means that to make the file smaller, the image editing software essentially throws away (or loses) pixels when you resave the file.
The concept is important to understand because when you compress an image, you can’t get those lost pixels back. When they are gone, they’re gone. That’s why when you reduce a digital photograph, it’s a good idea never to work on the original file. Before doing any image manipulation, always create a copy of the file. You don’t want to irreparably damage an original.
When you save a JPG file, your image editing software lets you decide on the quality. The higher the number, the more pixels stay in the image, and the better it looks. Higher number also mean larger file sizes. As you lower the number, the software starts throwing out pixels to reduce the file size. And image quality begins to suffer. Lowering the quality does have a considerable effect on file size. For example, if you take a 1.5MB JPG photo down to medium quality, the file size can drop by more than half to 600K or so.
The setting you choose depends on how you plan to use the image. For the best quality, leave the number at the maximum setting. If you plan to put the image on a Web site, use a lower setting. Most image editing software shows you how large the file will be before you save it, so you can determine at what point you want to make the trade off between a pretty versus a furry photo and longer versus shorter download times.