From the Adding and removing stickers from cards - Trello Help page
Trello Gold and Business Class subscribers can upload custom stickers by clicking the plus icon in the custom stickers section of the stickers menu and choosing an image file to be uploaded. Upload an image to create your own sticker. 128x128 stickers will show up the most clearly—other sizes will be letterboxed.
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I have never uploaded my own Trello stickers before but that would be a really fun project for me to do! I found this resource online. This person made a sticker pack available to add to your own account. Their dimensions are 200x200 and they look pretty good. Definitely make sure to make it a .PNG and make it transparent. Good luck to ya! Send some screenshots when you're done.
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