The first light picture ever taken!

I was reading a passage about 'selfie' and came across this sentence which was written on the back of the first selfie taken by Robert Cornelius in 1839. "The first light picture ever taken." I can't get the whole meaning. Actually I have a problem with the meaning of 'light' in this sentence. Any comments would be appreciated.

The two nouns 'light' and 'picture' stand in the same relationship as

oil painting
dry-point engraving
pencil sketch

The first noun describes the method of making the picture.

In this case light has been focused onto a prepared surface to make a chemical change which creates the appearance of a portrait.

National Museums of Scotland describe and display some early light pictures.