Tony Blair captured on mini-canvas: By whom?

The show features more than 1,500 postcards by 750 anonymous artists - many of them famous names.

Among the other celebrities included in the show are Chicken Run animator Nick Park and eccentric fashion designer Zandra Rhodes.

The other postcards are by up-and-coming artists, many of whom may well go on to be famous in their own right.

The sale regularly attracts huge crowds with people queuing all night, and around the block, to be first in for the 0800 start.

All the postcards cost just �35, with the money raised going to help students of fine art.

A landscape design for traditionalists - perhaps a Blahnik?

Buyers do not find out who has created their choice of postcard until they have bought it.

Then they can turn the card over to read the signature on the back.

This year, Bowie - who is also sponsoring the event through his website - has contributed two pictures.

Turner prize-winner Hirst - best known for making art from dead animals suspended in formaldehyde - has designed just the one.

Exhibition curator Emily Sargeant said: "His signed cigarette butts went for thousands, so his postcard could quite easily go for �5,000."

Elsewhere, works by film-maker Terry Gilliam and musician Brian Eno and shoe-maker Manolo Blahnik could also be worth a small fortune.

Pooch on a postcard - maybe a Bowie-wow

Trying to guess which postcard has been designed by whom will prove something of a conundrum.

They range from cartoons to abstracts, minimalist designs to graphic sexual poses.

There are even portraits of Prime Minister Tony Blair, his wife Cherie and former Chinese leader Mao Tse Tung.

British artist Billy Childish welcomed the anonymous format, saying it had banished the artistic ego.

Mystery woman - maybe a Zandra Rhodes creation?

"This brings out the best in art - it's very levelling.

"It's the anonymity - the egos get out of the way."

Experts advise buyers not to buy a picture because they think it looks like a particular person's style.

They say many contributors to the show are beginning to imitate each other.

Buyers are therefore better off just following their natural tastes and instincts.