You don't have to settle for pre-made WhatsApp stickers (or use a third-party tool) if you're sitting at your computer. The Verge reports WhatsApp has introduced a custom sticker creator for its web client and, within a week, desktop apps. Choose to attach a sticker and you can upload a custom image for editing. You can add emoji or text, crop pictures, cut out backgrounds and even slap existing WhatsApp stickers on top.
This tool won't help much if you mostly chat on mobile, where apps already exist to produce WhatsApp stickers. It might help if you frequently message people from your desk, though, and it could be particularly useful if you're either a digital artist (and likely have creative tools on your computer) or prefer to edit images with a mouse instead of your fingers.