Alex provided a link (see below) which shows some paintings from the 15th century and earlier. You can easily see how people, nowadays, will think these paintings may depict space craft. Although, specifically in the painting "Madonna with Saint Giovannino", I would expect if a UFO were depicted then it would be larger/more prominent. It actually looks like a smudge which has formed some unusual detail which could resemble a flying saucer.
The site also makes positive and agreeing remarks about Chariots of the Gods by
Erich von Daniken. So personally I would not give much credence to anything posted there.
I think von Daniken said of the famous Nazca lines, in Peru, that they were only possible to be constructed by an advanced technology (ie technology at the time the drawings were made, around 500BC, was too primitive). According to a wikipedia article researchers in 1994 were able to create fullsized replicas with simple technology in a matter of days.
http://www.visioninconsciousness.org...zations_26.htm