When an image is saved as a JPG (or JPEG) file, it is saved with some level of compression. Unfortunately however, the compression is “lossy compression” (unlike a PNG image which is bigger but where there is no loss in image quality). The lower the JPG compression is, the higher the image quality will be, and conversely, the higher the JPG compression is, the lower the image quality becomes.
Here’s an example of the same image saved using two different compression settings. We’ve zoomed in 200% to show the pixel information. If you look carefully, you can see 8×8 squares for the image on the right which is a clear indication of excessive JPG compression.
To avoid losing too much of the original data (pixel information) from the source image, it is recommended that you save JPG images with the highest quality (lowest compression) settings available in the software you’re using.
For Focus Magic, when you save an image as a JPG, it will save it with the highest quality (lowest compression) possible. If you open a JPG image and Focus Magic detects that it has a high level of compression, then you will be warned that the image you’re working with is potentially of a low quality and that your end result may not be as good as it could be. Therefore, whenever you work with JPG images, always try and work with the highest quality copy of the image that you can.
To read more about this, please refer to the Lossy compression article on Wikipedia.