I, an armchair art historian, was interested in looking at the history of how paint and pigment was manufactured over the years, and It struck me I had no idea of the earliest known painting. I am aware that the "last supper" is an oil on plaster experiment, thus it's decrepit nature, but i don't think it's the earliest.
I thought this would be the better place to ask, than /r/ArtHistory
This one. The Arnolfini Portrait. OK - probably not the first one ever, but the first significant oil painting. It's now in The National Gallery in London. A good book on early painting, before oils, for both the armchair art historian and the practicing artist is The Practice of Tempera Painting by Daniel V. Thompson Jr originally published by Yale in 1936 but available in a Dover Press reprint.
I found this article http://news.nationalgeographic.com/news/2008/02/080205-afghan-paintings.html it is a rather interesting read. Clearly oil painting was first developed in Asia but this may just be the earliest one found so far.