Pixelation effect essentially lowers the definition of images, while increasing the size of the individual pixels, or dots, that make up images.
If you need to pixelate your image, you can try PicPick and it's free.
Step1. Open the existing file in PicPick
Step2. Click Choose
Step3. Choose the area you would like to do pixelate
Step4. Click on the "Effects" button and then select the "Pixelate"option from the drop-down menu that appears.
Step5. In the "Pixelate" menu, drag the "Pixelate" slider to the right to add and increase the amount of pixelation applied to your image. Then click OK.
Check the result: