A profile picture is chosen at one size and seen at another. The settings screen shows it large; feeds and comment threads shrink it to a fraction of that. Choose without allowing for the difference and you end up with a picture nobody can make out.
Shrink it before you decide
Once you have a shortlist, display each one small and look again. Zoom the browser out, open it on a phone — the method does not matter. That one step removes the candidates whose detail turns to mush.
Put the other way round: the more finely worked an image is at full size, the less it tends to suit a profile picture. Make the decision at the small size.
Expect a circular crop
Most services crop profile pictures to a circle, so the corners of your square never appear. Anything important in the corners is lost. Some services use a rounded square instead, but either way it is safest to assume the corners go.
A subject near the centre with room around it is the easiest to work with. A tightly cropped subject also survives a circle well. What to avoid is a composition where the point of interest sits near an edge.
A simple background does more work
The first thing lost at small sizes is the boundary between subject and background. The closer the background is to a single flat colour, the better the shape holds up. If you prefer scenery or a pattern, pick one with few elements and clear contrast.
The direction of the face changes the impression
A figure facing forward reads as steady and straightforward; one turned slightly away reads as more animated. If you keep separate accounts for work and for everything else, changing only this is enough to tell them apart.
Animals, objects and scenery are an option too. They fix the impression less firmly than a person does, they identify you less, and they age more slowly.
Match across services, or vary deliberately
Match them when you want to be recognised as the same person; vary them when you want the accounts kept apart. If you match, check the picture as both a circle and a rounded square, since services differ.
About Visual Hub profile pictures
Our profile pictures are square, composed with the subject towards the centre because a circular crop is expected. The asset page displays them as circles, but the file you download stays square, so it still works on services that crop to another shape.
Alongside people we have cats, dogs, birds, scenery, patterns and objects. Use those when you would rather not put a person up.