Right shape. Right slot.
A profile photo is not a header. An ad is not every organic post. Each guide names the specific publishing surface its source covers.
Social size guides
The personal profile and background are separate slots. Keep important banner content away from the profile-photo overlay and inspect the preview on your own account.
X
These sizes concern profile assets, not post images. The interface can crop the header differently across screens, so keep the subject near the middle.
YouTube
The current help page distinguishes computer and mobile upload limits. A common older size recommendation is not repeated here as the current official value. Custom thumbnails require an eligible verified account.
This guide is deliberately scoped to standard image ads. Pinterest re-encodes uploads and may add a faint background layer to distinguish very light or dark images from the feed.
Discord
This is a custom emoji guide, not an attachment-size chart. You need the relevant server permission to add expressions. Backdropless produces a still image and does not preserve animation.
Mastodon
Individual servers can differ from the documented defaults. Confirm the upload interface on your server and preserve an original in case you later move your account.