Create your pixel sticker in 60x60 pixels or smaller. I'll use this bear pixel as an example.
Add a white outline of 1-3 pixels, depending on the size of your sticker.
1 pixel border
2 pixel border
Then add a 1 pixel outline in a darker color. Save as a transparent PNG.
Making it like this will make the pixels look more sticker-like and having a darker outline will make it stand out just in case it gets displayed on a white background.