Listen! what i think jpg format file is one of the richest image file. when you change pixel values in png format file, the value is exactly being changed for that pixel, but the thing is when you try doing samething with jpg file it does not do so. You would better know by performing a simple experiment.

make an image of 150*100 in MSpaint. put red color on first 50 rows, green on next 50 rows and blue from 100th to 150th row and make the last 50 rows half white and half black in color. like (1,150) to (50,175) white color, and (51,175) to (100,200) black color. then save this file in jpg format. and then open the saved image. You would notice something very interestng at the borders of the colors. You would get your answer. Do try it.

What I think JPG format file is one of the richest image file. When you change pixel values in PNG format file, the value is exactly being changed for that pixel, but the thing is when you try doing samething with jpg file it does not do so. You would better know by performing a simple experiment.

Make an image of $150*100$ in MSpaint. Put red color on first 50 rows, green on next 50 rows and blue from 100th to 150th row and make the last 50 rows half white and half black in color. like (1,150) to (50,175) white color, and (51,175) to (100,200) black color. then save this file in jpg format. and then open the saved image. You would notice something very interestng at the borders of the colors. You would get your answer.