Remember once and for all, that if you are constantly peresohranyaete JPEG image over the original, it can lead to loss of quality - and restore it back - you will not succeed. Pixels are solid, and the image will be "artifacts" (this tells us encyclopedia irregular pattern of pixels distorted color and / or brightness). also raznotonovost image is removed, all similar colors are likely to be perceived as a result of the same.
principle, again, Format good for the web, but it has one significant disadvantage to other Formatami intended for the same web (such as PNG and GIF) - it does not support transparency.
Formata compression of this place of this principle algorithms (taken from Wikipedia):
1) convert the image to the optimal color space;
2) Sub-sampling component color averaging groups of pixels;
3) The application of discrete cosine transforms to reduce redundancy of the image data;
4) quantization of the DCT coefficients of each block with the use of weight functions optimized with human readability;
5) encoding the resulting coefficients (image data) using the algorithm group and Huffman coding to remove redundancy.