The answer is TinyPNG free service:
TinyPNG uses smart lossy compression techniques to reduce the file size of your PNG files. By selectively decreasing the number of colours in the image, fewer bytes are required to store the data. The effect is nearly invisible but it makes a very large difference in file size!
Difference between images:
Below are two photos of my cousin. The left image is saved as 24-bit PNG directly from Adobe Photoshop. On the right is the same image, processed by TinyPNG. Spot any difference?
In the above image the file size is reduced by more than 70%. I have excellent eyesight but can’t spot the difference either! Use the compressed image to save bandwidth and loading time and your website visitors will thank you.
Reference link: http://tinypng.org/