How do you do it on MacBook or kindle?

Save a gif (must be small in file size and no bigger than 500x500

and then upload it as your pfp.

Just upload a gif as your PFP. Be careful of filesize though as it won't be able to be uploaded if it is too big

Edit: Ninja'd (60 second rule)

you must download it to your computer first and then upload it to scratch

There are some websites that allow you to create gif (eg. ezgif, gifmaker etc.). After making your gif, hover over your pfp on your profile page and upload the gif. Do note that the dimensions should be no bigger than 500x500 and that the file size limit is 500KB.

Also, here are some good places to make your pfp a gif:

• Pixilart - You can draw pixel art and create frames for a gif.
  
• Photomosh - Provide the site with your pfp and they will glitchify it. You can choose from many different settings to glitchify your pfp.
  
• GIPHY - A place where you can find gifs and make gifs.
  
• imgflip - Another place to make gifs
  

If you want to make your PFP a customisable animated GIF, I would recommend either using a website that can do just this (see above), or using a free image editor (GIMP for instance) for making great GIFs. My PFP was made using GIMP!