LinkedIn personal banners: 1584×396 pixels, 4:1, JPG or PNG, up to 4MB. Company pages use 4200×700 — and the 1128×191 figure in older guides is out of date for both. Drop an image; it returns at exactly 1584×396, ready to upload without LinkedIn's own cropper mangling it.
The rule that matters most: the bottom-left is not yours. The profile photo overlays roughly a 568×264 region there, desktop and mobile alike — anything placed behind it disappears. Compose to the right two-thirds or dead centre; the crop position slider shifts the source to clear the overlap.
Fitting photos to 4:1: the ratio is very wide, so most images lose significant top and bottom. Landscape sources with the subject placed high survive best; wide scenes, textures and skylines take the trim gracefully; group shots mostly don't.
Text in banners: large, and right of centre. On phones the banner renders around 200 pixels tall — small text becomes decoration.
JPG encoding and the 4MB ceiling are handled automatically, with real file sizes shown. The cropping runs in this page on your device — the only upload that ever happens is yours, to LinkedIn.