Digging about last night on the National Portrait Gallery website, I found two very tasty posters at reasonable prices – exhibition posters for Twiggy and Beatles to Bowie: The 60s exposed.
The Beatles to Bowie poster is from the exhibition that took place over 2009/2010 and features a 60s image of David Bowie by David Wedgbury. Sized at 508mm x 762mm, it's just £6 online.
The Twiggy image is by Barry Lategan from 1966, which was used to promote Twiggy: A Life in Photographs, also over 2009 and 2010. It's slightly more expensive at £10. Alternatively, Heals does a version professionally framed for £100. I know which one I would go for.