JPG files are lossy-compressed images. This means that when you save a photo as a JPG, some detail is lost from the original image each and every time you resave it as a JPG. When you take photos as JPG files with your digital camera, you may not notice this loss of detail, especially if you use a fine or super-fine low-compression mode. However, open a JPG file in photo editing software and resave it as a JPG, and you may begin to see some banding or noise, especially if the compression is reset to high. Do this often enough and your image becomes a deteriorated grainy or blurred version of its former self.
I once had someone tell me that they had resaved their JPG file so many times that all quality was completely destroyed.
You'll see that the kitten on the right is starting to lose quality after being saved several times in a JPG format.Viewing the file, copying it to another location, renaming, or any action that does not involve opening the file in an image editor and saving a changed image will not affect the image's clarity.
Solution
Before you modify JPG images with your photo editing software save an uncompressed original as a TIF. BMP or native Photoshop PSD file. Then edit and save a .jpg file for web or email.
Not only is it a good idea to have a high quality backup, this uncompressed, unaltered image is always available in case you need it again or heaven forbid, do some editing that just didn’t work right.
More information on digital compression
