How to prepare new custom emoticons?

[sofos]

Hi everyone, I downloaded some emoticons from freepik (PNG - px 512 x 512 - 155 DPI) that I would like to add to the custom emoji.

I would like to know what is the correct procedure to insert them, as they are too large, reducing them to 20 px excessively lose sharpness.

🙂 Thanks in advance!