Sticker 512
Scales images so the longest side is exactly 512 px and saves them as PNG, transparency kept. That is what Telegram's @Stickers bot expects for static stickers.
In any imageOut PNG · 512 px longest sideRuns in your browser
Drop images hereor click to choose · paste works too
Telegram limits
One side 512 px, the other 512 px or less, at most 512 KB. Files over the size limit are marked in the list.
Next step
Send the PNG files to @Stickers. Details in the Telegram sticker guide.