In short: Discord stickers need exact dimensions (320×320), a real alpha channel, and a file under 512 KB — and most "remove the background" tools don't check any of the three, so the upload looks fine locally and fails at the server. RemoveGifBG has Discord-specific size presets (128×128 emoji, 320×320 sticker) with real per-pixel alpha, so the export passes the first time.
You want a custom sticker or emoji for your server. You take a GIF or a piece of art, run it through a background remover, and try to upload it — and Discord bounces it. The image looks transparent when you preview it. The rejection usually comes down to one of three things Discord actually checks that a generic background-removal tool doesn't: exact canvas size, real per-pixel alpha (not just "looks see-through" in a preview), and file size under the limit.
RemoveGifBG has built-in size presets for Discord — pick Discord Emoji (128×128) or Discord Sticker (320×320) as the output dimension before processing, so you're not resizing a finished cutout afterward. The alpha channel is real per-pixel transparency, not a preview trick, so it uploads correctly.
For a full emoji or sticker pack, batch up to 5 files in one job and download them together as a ZIP — useful if you're rolling out a themed pack for a server relaunch or event.
Discord's actual limits (verified against Discord's own docs)
See also: Discord sticker tool