ca. 1722 Duchess of Marlboro with Lady Diana Spencer by Maria Verelst (location unknown to gogm)
According to Avicenna in a posting to The Royal Forums of 16 August 2008, "...Sarah, Duchess of Marlborough, together with her granddaughter Lady Diana Spencer. Painted by Maria Verelst following the death of the Duke of Marlborough." The Duke died in 1722.
According to this Lady Diana's Wikipedia article, she was considered as a bride for Frederick, the then Prince of Wales, but as put by Jo of Palatine's posting on The Royal Forums of 4 August 2008, "Queen Anne had been long dead then. And at that time the Hanovers had comfortably settled into Britain while their possessions in Germany were at risk from their neighbors, notbaly the Prussians. So it would have made politically absolutely no sense to marry the Prince of Wales to a mere British nobleman's daughter when he could have a Saxe-princess to keep the Prussians in check." Frederick married Augusta and Diana married John Russell, the 4th Duke of Bedford. This Lady Diana Spencer's Wikipedia article is here.