Pixellation is very hard to fix.

The image shown is of a high-resolution photograph. If that photograph is reduced when sending via email, or by pasting into a low-resolution template, the effective dots per inch or resolution of the image, is reduced, creating the squared-off, pixellated effect. While this can be improved by blurring at high resolution as shown in the third image, although this removes the jaggies, it cannot replace lost definition.