JPEG images - ôJPEGö stands for ôJoint Photographic Experts Groupö - reduce the file size by discarding extra data not essential for the display of the image. The bigger the compression, the smaller the image becomes.
Because it discards data, the JPEG algorithm is referred to as ôlossyö. (TIFF Group 4 compression and PNG images are ôlosslessö.) Once an image has been compressed, it is no longer identical to the original image! JPEG images are automatically decompressed when they are opened.
In most cases, nobody can notice the difference between the original image and an image with minimal compression. However, image quality can degrade substantially as the compression ratio increases.
Choose an appropriate trade-off between the image quality and the amount of compression!