Whenever you work with a picture, it's best to work with a copy of the original. Every time you save a jpg image, you lose quality; if you save a jpg repeatedly, your picture can look mighty bad pretty quickly.
JPG is a lossy format. "Lossy" means "it loses." Here's a picture of my Guide Dog Puppy, Galaxy. This is a copy of the original image.
I took this picture of Galaxy and saved it out, calling it 2.pg. I then opened up 2.jpg and saved it as 3.jpg. I did this until I had 26 images. I chose the smallest file size in the compression settings to make this example more obvious. (You'll need to click on the below image of images to see a large version of it.)
The first image is the original copy. By the sixth image, you can see funny discoloration on Galaxy's right, rear leg and back. That discoloration looks worse in the 11th and 16th iterations. By the 26th image, her leg and back look pretty bad and her face starts to get messed up.
To make this even more obvious, I took a different image and repeated the procedure at an even lower file size setting. This picture is of my daughter holding, Galaxy, our Guide Dog Puppy, a few days after we got her.
Again, you'll need to click on the image to see a larger version.
This time around, the degradation is bad just by saving it once. The image is so bad with the first save, that it's difficult to tell that the remaining images get worse.
Lossy formats are great when you're concerned about file size. And, if you don't try to minimize file size, you can resave an image a few times before it looks bad.
Lossless image formats don't have this problem. They don't lose information and they don't degrade. However, they're bigger. Lossless formats include gif and png.