Walking along Great George Street, you'd be forgiven for passing number 5a without a second thought. At first glance, it seems like just another elegant Georgian townhouse in the centre of Bristol converted into offices.

Few passers-by can have any idea that stepping through the heavy old door is a gateway to some of the world's greatest natural wonders.

The building is home to one of the world's largest collections of photographs of the natural world.

Taking in everything from the mightiest blue whale to the peskiest mosquito, the Nature Picture Library contains a staggering wealth of images – many of which are the most stunning photos you've ever seen.

With 150,000 pictures currently available on the Nature Picture Library's website and 2,000 being added each month, the modest office close to the bustling shops of Park Street provides illustrations for countless newspapers, magazines, books and greetings card companies around the world.

In fact, wherever you see a picture of animals – from advertising billboards to jigsaw puzzles – there's a decent chance it's come from this little office.

"Bristol has long had a vibrant community of wildlife photographers thanks to the fact that the city has played host to the BBC's Natural History Unit for more than half a century," explains the Nature Picture Library's managing director Helen Gilks as she leads the way past filing trays bursting with transparencies all waiting to be scanned on to a mammoth computer archive.

"We originally started up within the Natural History Unit, but we set up as an independent company in 2002.

"We have lots of pictures from BBC wildlife documentary shoots – not just animal photos, but also behind-the- scenes shots – and these days we have many thousands of pictures taken by a wide range of wildlife photographers from around the world.

"Our archive covers almost every kind of animal and plant you can think of, and we recently purchased the Bluegreen archive of maritime-themed pictures which contains about 40,000 images, with new shots being added all the time.

"When I started in 1995, it was all transparencies, we had row upon row of filing cabinets. But these days, everything is digital.

"All our images are stored on vast computer hard-drives, though we do still have a few thousand pre-digital pictures being scanned."

Freelance photographers can submit their work to the archive and receive half the money from any sale of their images. But pictures have to be of an exceptional standard – and must fill a rare gap in the archive. "It's very difficult to come up with creatures that we don't already have on the archive" says Helen, "but there's always scope to photograph interesting behaviour that we might not have covered – as long as the photographer has sufficient patience and perseverance.

"I normally advise photographers to take a good look through our online archive before even considering a submission to check we don't already have something similar.

"Different markets are interested in different kinds of pictures. Wildlife books might be able to make use of interesting shots of animal behaviour, but greetings cards companies are always looking for 'clean' and cute images.

"The card companies are always after new images of the iconic animals – things like pandas, polar bears, foxes and frogs.

"Sometimes the best pictures aren't taken in the wild at all, but have clearly taken a lot of work to set up.

"We recently added one to our system of moles burrowing underground. The photographer had to build an entire glass-fronted mole colony to get the shot.

"That might have taken weeks to set up, but it makes a unique photograph which is bound to sell well."

But pictures don't have to be cute to be successful.

"Funnily enough," she laughs, "one of our best-selling pictures is an amazing shot of a mosquito extracting blood from a person's arm."

The vast majority of the company's business never directly involves the public, but they do sell prints to wildlife lovers directly through their website.

"Most of what we do is business-to- business," Helen says, "so it's not surprising that people don't know we're here. But the prints are becoming increasingly popular.

"Whether it's the maritime images from the Bluegreen collection or the wildlife and animal shots from the rest of our archive, they do make wonderful images for around the home."

For more information about the Nature Picture Library, visit the website at www.naturepl.com