What is the best picture for a photo mosaic?

Your picture is cropped to a tall 4 by 5 shape and divided into a grid of 40 tiles across and 50 down, 2,000 in all. Each tile takes one colour from the part of the picture it covers. People's photos are placed where their own colours fit best, and each is lightly tinted toward its tile's colour.

So the mosaic can only show what survives at 40 by 50. Every rule below follows from that one fact.

Open the photo on your phone and hold it at arm's length, or shrink it until it is the size of a postage stamp. If you can still tell at once who or what it is, it will make a good mosaic. If you have to look closely, choose another.

One subject, close up. A head-and-shoulders portrait gives a face several hundred tiles to work with. Two people work if their heads are close together.

Clear light and dark. Faces lit from a window or photographed outdoors on a bright, cloudy day have shape. Direct flash flattens a face into one pale area with little in it.

A plain background that differs from the subject: dark hair against a pale wall, a light shirt against a hedge. The outline is what people recognise first.

Bold graphics. A logo, a crest, a large numeral or a single thick letter often reads more clearly than any photograph, because it is already made of simple shapes.

Group photos. Ten people across the frame leaves each person four tiles wide and each face narrower still, which is not enough to recognise anyone.

Fine detail: small lettering, thin lines, patterned clothing, lace, distant buildings. Anything narrower than a tile disappears.

Busy backgrounds such as a crowd, a bookshelf or foliage. They turn into scattered colour that competes with the subject.

Very dark pictures. Tiles in dark areas are tinted dark, so the photos placed there are harder to see when someone zooms in. A night-time or candlelit photo hides a large share of the contributions.

The crop to 4 by 5 is automatic and is centred on the most prominent part of the photo. That is usually right for a portrait. For a wide photo it may not choose what you would, so crop it yourself first: upright, with the subject filling most of the frame and a little space above the head.

Sharpness and file size matter much less than you might expect, since the picture is only read as a coarse grid. An old, small or slightly soft photo is fine if it passes the ten-second test. A print photographed with a phone is fine too: lay it flat near a window, with no flash and no frame glass.

You can change the picture later from your owner page. The photos people have added stay, and the tiles rearrange to form the new picture. It is worth trying two or three candidates in the first few minutes, before you share the link.