She must have been having so much fun.

Chuckling with delight, Madeleine McCann dangles her feet in a pool and poses for a holiday snapshot.

Less than eight hours after it was taken, Madeleine disappeared. So yesterday this became the last, poignant picture of the missing four-year-old - released by her grief-stricken parents in their desperate campaign to bring her home.

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This newly released photograph of Madeleine shows her laughing with her family on the day of her disappearance

It shows Madeleine with her father Gerry and two-year-old sister Amelie, sitting beside the pool in their Algarve holiday apartment complex. Her mother Kate is behind the camera with Amelie's twin brother Sean.

It could hardly have captured her character more perfectly - a cheeky little Miss in a pink summer dress and floppy hat, stealing the limelight as usual with that trademark smile.

That was at 2.29pm on May 3, a few days after the family checked in to an apartment in Praia da Luz and just nine days before her fourth birthday.

Kate McCann with Sean yesterday

And last night, another picture of Madeleine was projected onto London's iconic Marble Arch to bring her plight to the attention of the city.

During the day of her disappearance they had all gone for a walk, played tennis, and taken some more snapshots for the family album before relaxing by the pool.

Some time between 9.10pm and 10pm, an abductor is assumed to have snatched her from her bed while her parents dined nearby.

Three weeks on, there has been not a single trace of her whereabouts, no breakthrough in the investigation, and no clear indication that she is even still alive.

In the evening her parents gave her a bath, took out her trendy hair bead and washed her hair. Then they tucked her up alongside Sean and Amelie before kissing them all goodnight.

Mr and Dr McCann - he is a consultant cardiologist, she a GP - remain convinced she is alive, and have vowed that they cannot even consider going home to Leicestershire until they find her.

But yesterday, for the first time during the remarkable campaign they are running in parallel with the police operation, they made known their frustration at the investigation's lack of progress.

The 38-year-old couple met Portuguese police chiefs to express their concern at the "apparent slowness" of the hunt.

A source close to the family said: "At every stage throughout this they have been feeding their own questions and concerns back to the investigating authorities through British police and in turn the Portuguese authorities.

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A photograph of Madeleine McCann is projected onto London's iconic Marble Arch

"While they remain content with the overall thrust of the investigation they do at times hope that their questions and concerns are addressed more quickly."

Yesterday the latest Portuguese initiative - a plea to the kidnapper from the Bishop of the Algarve - stalled almost immediately.

Bishop D. Manuel Neto Quintas appealed to Madeleine's abductor to phone him to arrange Madeleine's return - but he failed to supply a phone number.

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Madeleine's father Gerry with her twin sister Amelie  (centre) and Madeleine (right)

 as the family relaxes by the pool. Hours later their holiday would turn into a living nightmare that appears to have no end in sight

In other developments, new leads have emerged from more than 300 holidaymakers' photographs that have been sent in to an appeal for pictures that might reveal the abductor.

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Gerry McCann made a moving visit to a shrine in Madeleine's honour in their hometown of Rothley, Leicestershire, recently

Officers from the Child Exploitation and Online Protection Centre had appealed to tourists who visited Praia da Luz in the two weeks prior to the kidnap to send in their holiday pictures.

Strangers in the background of pictures are being cross-referenced with an image of known paedophiles using facial recognition software.

A spokesman for CEOP would not comment on the detail of the new leads.

It also emerged that the McCanns have been offered the use of private jets to make trips around Europe in their continued campaign to find Madeleine.

Early discussions are under way with Warner Brothers cinemas to put posters in every cinema and to play an appeal video of Madeleine before every movie.

Frustration is growing as the weeks slip by in the search for Madeleine