Design for a circle, not a rectangle
LinkedIn masks your photo into a circle at roughly 48 to 160 pixels in most surfaces. Anything happening at the corners is invisible, and anything smaller than a few pixels — a logo on your shirt, a lanyard, text on a mug — turns into noise.
Crop so your head and shoulders fill about 60% of the square, with your eyes on the upper third line. That single change makes a photo look professional more reliably than any filter.
Wardrobe, background and colour
Wear solid mid-to-dark tones. Fine patterns alias badly at small sizes and busy prints pull attention away from your face. Pick a background that contrasts with your hair and clothing so your silhouette separates cleanly.
Avoid pure white backgrounds if LinkedIn's page background is also white — the photo will feel like it floats. A light grey backdrop reads cleaner in the feed.
Expression is the whole ballgame
A slight, genuine smile with engaged eyes consistently outperforms both a neutral corporate stare and a wide grin. Recruiters are making a fast trust judgement, and warmth reads as trustworthiness.
Keep the natural asymmetry of your face. Perfectly symmetrical, perfectly smooth portraits trip the same instinct that makes people distrust stock photography.