There's an interesting article on the 24 Hour Museum site by Jacob Simon from the National Portrait Gallery on the portrayal of disability in art. Several of the artists mentioned also feature in our collections including Zoffany, Hogarth and Reynolds.
This got me thinking about the portrayal of disability in our own collections, and wondering how much attitudes have changed in the time since these magnificent pieces were painted. Just off the top of my head I can think of the blind man in Holman Hunt's 'The Finding of the Saviour in the Temple' (see image), and three depictions of Nelson (by West, Drummond and Maclise) who at the time of his death was missing an eye and an arm.
Benjamin Robert Haydon considered himself near blind but still managed to produce several works including 'Christ Blessing the Little Children' which was commissioned to decorate a chapel for the blind (this linked page includes his son's description of Haydon wearing several pairs of spectacles at once).
There is also the theory that JMW Turner had increasing problems with his sight as he aged - colour blind from an early age and then cataracts in later life (check out this Guardian article). Works like The Falls of the Clyde might be seen as the result of a condition, though we probably wouldn't describe Turner as disabled.
I guess the most famous modern representation of disability in art is Marc Quinn's wonderful statue, 'Alison Lapper Pregnant', which stands in Trafalgar Square. It seems that attitudes really have changed.