The safe zone is the point
A banner is not displayed the way you upload it. Facebook shows a page cover at one shape on desktop and crops the sides hard on mobile. YouTube channel art is the extreme case: you upload 2560×1440, and the only region guaranteed visible on every device — phone, desktop and television — is a 1546×423 strip in the middle. Everything outside it is decoration that some of your audience will never see.
Put your name, your logo or your call to action outside that strip and a large share of visitors get a banner with the important part missing. It is the single most common mistake in this category, and it is invisible until someone tells you.
So where a platform has a published safe zone, it is drawn on the preview as a dashed outline with a label. It is an overlay on the preview only — it never appears in the file you download.
Working within an extreme aspect ratio
A LinkedIn company cover is 1128×191 — nearly six times wider than it is tall. Almost no photograph survives that crop intact, which is why the good banners tend to be textures, gradients, patterns or wide landscapes rather than pictures of things.
If you must use a photograph, the crop-to-fit fill with the focal point dragged to the subject is your best option. If the picture cannot survive, the blurred or solid fill keeps the whole image visible inside the band, which reads far better than an arbitrary slice through someone’s face.
Start with the largest source you have. A banner is displayed wide, so a small image scaled up to fit shows every flaw.
File size limits some platforms enforce
YouTube rejects channel art over 6 MB and thumbnails over 2 MB, and the rejection message is not always clear about which limit you hit. If a banner comes out large, compress it before uploading — the compress tool will hit an exact target.
Most other platforms are more forgiving, and all of them re-encode what you give them regardless. Uploading exactly the stated dimensions avoids one extra generation of their compression.
Everything here happens in this browser tab. The image is decoded, redrawn and re-encoded by the Canvas API on your own device, and the result is assembled in memory and handed to you as a download. Nothing is uploaded, which you can confirm in about fifteen seconds with the network panel open.