Apart from its presence in ancient cave paintings and mythologies, the presence of saucer shaped flying objects are also visible in many famous European medieval artwork. Following are some famous controversial paintings of such kind:
These images of two crusaders date from a 12th century manuscript Annales Laurissenses" (volumes/books about historical and religion events) and refer to a UFO sighting in the year 776, during the siege on Sigiburg castle, France.
These two tapestries were created in the 15th century. Both depict the life of Mary. Hat shaped objects can be clearly seen in both tapestries. The one on the right is entitled "The Magnificent". Both are located at the French basilica Notre-Dame
The above painting is by Carlo Crivelli (1430-1495) and is called "The Annunciation" (1486) and hangs in the National Gallery, London. A disk shaped object is shining a pencil beam of light down onto the crown of Mary's head. A blow up of the object can be seen below:
The above is a painting on wood drawer from furniture kept at the Earls Doltremond, Belgium. Moses is receiving the tablets and several objects in the sky are seen near by. Date and artist unknown.
The above picture depicts Jesus and Mary on what appear to be lenticular clouds. The painting is entitled "The Miracle of the Snow" and was painted by Masolino Da Panicale (1383-1440) and hangs at the church of Santa Maria Maggiore, Florence, Italy.
This image is by Flemish artist Aert De Gelder and is entitled "The Baptism of Christ" It was painted in 1710 and hangs in the Fitzwilliam Musuem, Cambridge. A disk shaped object is shining beams of light down on John the Baptist and Jesus.
The opponents of ufology states that, explanations for these round shaped objects would vary from painting to painting. Many experts referred to ‘Abstract’ & ‘modern art’ and stated that the meaning of these objects might vary from artist to artist & may not necessarily carry the same meaning in all these pictures. Hence concluding that all these artists encountered flying saucers in their life and then drew the picture in a symbolic way to alert the future generation, would be a stupid assumption to hold.
The usual explanation for these mysterious objects concludes in a similar pattern and to the opponents, the obvious answer is: ‘Religious Symbolism’! In 2004, Diego Cuoghi (a renowned italian art historian, writer & artist) came with a detailed explanation of all these paintings. In his publication, The Art of Imagining UFOs, in Skeptic, he tried to portray the symbolism of many famous artist of that time and presented a very rational meaning behind most of these paintings.
Here is a link where you will find some translated portion of The Art of Imagining UFO's: http://www.sprezzatura.it/Arte/Arte_UFO_1_eng.htm