Compression methods can be lossy,
when a tolerable degree of deterioration in the visual quality of the resulting image is acceptable
or lossless,
when the image is encoded in its full quality. The overall results of the compression process, both in terms of storage savings – usually expressed numerically in terms of compression ratio (CR) or bits per pixel (bpp) – as well as resulting quality loss (for the case of lossy techniques) may vary depending on the technique, format, options (such as the quality setting for JPEG), and the image contents
As a general guideline, lossy compression should be used for general purpose photographic images.
whereas lossless compression should be preferred when dealing with line art, technical drawings, cartoons, etc. or images in which no loss of detail may be tolerable (most notably, space images and medical images).