It's actually not the file type that pixellates an image. It's the size and resolution. If you resize a small image down even smaller it'll be pixellated in PNG or JPEG format. The only way to minimise pixellation is to downsize a large crisp resolution image that's say of a size greater than 2400 x 2400 px down to something as small as 500 x 500 px in size, and preserve the image quality when you save without degrading.

Photoshop and GIMP allow this in PNG and JPEG format.

I do the resizing thing all the time with my own art in JPEG or PNG format.

You can see a bit of pixellation in the above image, but I thinks that's actually from not colouring over the lines. It will always leave a jagged edge on a digital image regardless of sizing, file format, and image resolution, unless you colour half on the lines, or colour over them.

I made this mistake in a recent art piece. It's not visible in the final coloured image because I resized down from a high-resolution 4000 px+ image to 1200 px.

It's easier to hide the flaws.

Oooh! Well now the moon looks nice!
Did you save the file in jpg this time? The pic is a bit more pixelated.
I always save my pics in png. I feel like it is the best way to save the quality and still be able to upload pics on internet.

Amazing job!