A few people asked about JPEG compression so here is an image at various compressions. This is reprinted from a page I wrote back when JPEG was first announced.
At 80% compression, the bird in the following image looks bad with pixelation errors easy to spot.
Because the bird is an unfamiliar image to most people, they do not notice the image destruction at 50%. I can see it because I know how good the original image looks. You can see the damage by comparing the 50% image with the zero percent image.
If you try the same JPEG compression levels with the image of a familiar human, the image quality destruction becomes obvious around 30%.
Zero JPEG compression
This picture looks perfect. Internally the JPEG system has already removed some of the original detail.
20% JPEG compression
At 20% compression, there is a little colour shift around the bird's tail. That is because JPEG compression reduces the range of colours in the image.
50% JPEG compression
50% compression shows reduced colour and distortion along high contrast edges.
80% JPEG compression
80% compression shows the square pixellation effect of JPEG. JPEG tries to fake image detail by overlaying areas of thin fine detail on broad areas of less detail.
90% JPEG compression
90% compression gives the image an impressionistic look that is not nice.
95% JPEG compression
At 95% compression you can clearly see the square constructs JPEG uses to fake an image. The higher resolution images have a lot more squares but not as many as the original image.
99% JPEG compression
AT 99% compression the JPEG is down to the same number of colours you see in a GIF file and no longer has enough squares left to fake an image.