Why can't I save all my pictures in the JPEG format?

JPEGs are great because the file sizes are usually very small. Unfortunately, JPEG compression is a "lossy" format, which means some information is lost every time you create a JPEG. In fact, every time you uncompress a JPEG image for editing and then re-compress it again for saving, the image degrades further. After a few editing sessions of the same image, your image will no longer have the integrity of the original.

To avoid image degradation, keep all your picture assets (your original scan and other picture elements intended for further manipulation) in native Photoshop format (.psd) until you are ready to publish. The PSD format is "non-lossy."

The consensus used to be that all files for print must be saved in the TIFF format in order to retain the integrity of the original. But since the advent of the Internet, all that has changed. For print, save your JPEG files using Photoshop's default "maximum" setting of "10" (the highest setting of "12" is not recommended as file size increases dramatically with virtually no improvement in image quality). The "maximum" JPEG compression is so good that the human eye can not tell the difference between the original and the compressed version. But the file size is amazingly small (a 10 MB file can be compressed to less than 1 MB).

Line art (b&w bitmapped images with no tonal changes) intended for print should always be saved as TIFFs. These files are generally smaller and very efficient anyway.

By the way, JPEG stands for Joint Photographic Experts Group, the inventors of this incredible format.

Related article: GIFs vs JPEGs