Adolf Hitler's stint as a jobbing painter has always been rather overshadowed by his subsequent career in politics. However, nearly a century after the future Führer was rejected by the Academy of Fine Arts in Vienna, 13 of his watercolours go on show today in a London art gallery.

They are the star attraction in a new exhibition by Jake and Dinos Chapman, the Young British Artists famous for showing mannequins of children with genitalia instead of faces and for Hell, a series of nightmarish Hieronymus Bosch-style dioramas arranged in the shape of a swastika, which was destroyed in the Momart warehouse fire in East London in 2004.

Bought anonymously from collectors around the world for a total of £115,000, the Hitler watercolours are mostly plodding landscapes and a few smaller studies. The Chapman brothers have transformed them — the gallery uses the word “annihilated” — by painting rainbows, psychedelic skies, floating lovehearts and smiley faces into the background of each picture. The resulting work is now available as a job lot for £685,000. The brothers said that they were expecting angry reactions to the work but denied that it was offensive or that they were profiting from Hitler's notoriety.

The show, at the White Cube gallery in Mayfair, is called If Hitler Had Been a Hippy How Happy Would We Be, drawing on the joke that the Second World War and the Holocaust might not have happened if Hitler had been more fulfilled as a painter.

During his time as a struggling artist in Vienna between 1910 and 1913, Hitler painted more than 1,000 paintings by his own recollection.

Originally sold for a handful of coins on the street and in beer halls, the value of the pictures increased sharply after he rose to power in the 1930s. They remain highly collectable.

Tim Marlow, director of exhibitions at the White Cube, insisted that the Chapman brothers' project was not glorifying the work. “There's no question about them paying homage to them,” he said. “These are very bad paintings - abject paintings - and Jake and Dinos have now annihilated them. We bought them anonymously because the last thing we wanted was to increase the market for Hitler's work.”

The original paintings are so “bland and benign” that they give no hint of the artist's monstrous imagination, the Chapman brothers said.

They have attempted to use them to explore one of their favourite themes: their obsession with the psychology of the artist to the exclusion of the art itself.

Jake, the younger brother, said: “It's endemic to the way people read art, to look for something in a work that's an indicator of some kind of symptomatic trauma or a revelation of the artist's inner self, rather than trying to identify how the work works.

“When you look at the Hitler paintings you try to work out if this person was ill or mad or whether this is in some way axiomatic of someone who will go on to kill seven million people. [But] the drawings of themselves aren't offensive.”

James Smith, chief executive of the Holocaust Centre in Newark, said: “Hitler's mediocrity and blandness as an artist illustrate that it takes neither a genius nor a psychopath to organise genocide, and as such, his paintings do have some value as historical artefacts. Painting over his originals to make a point about the past and its relation to the present is probably the most appropriate form of vandalism I have encountered.”

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