I think these pictures could have been sketches in chalk or, indeed, pencil.They would not have been wax crayons,as these were invented in the late 19th century..

Pencils were certainly available at this time;have a look here at the history of Cumberland pencils.

http://www.pencils.co.uk/p_history.asp

As to the subject matter of the pictures in the gallery at Pemberley, they may have had classical allusions, rather like the work of Nicloas Poussin.Below is a link to some of his works, including my favourite of all time- The Dance to the Music of Time held by the Wallace Collection in London.

The Darcy family had a large and old library, and I have no adoubt that their art collection would likewise have been the work of many generations, and would have contained many old masters.

As Elizabeth - like most girls of the late 18th early 19th cnetury-had not been the benficiary of a classical education, she would not immediatly understand this type of picrute.(IMHO)