A lot of these things I find on Wikipedia which is a good start. However, most of my knowledge comes from books.
But if you want a good online source, I'm not sure.
But I will tell you this. You can't learn about Roman Catholicism unless you learn about the Orthodox Church because they used to be one and the same until the Great Schism in the Eleventh Century. Until then there were two Popes.
You will also benefit from the learning about Oriental Orthodoxy because they split from Rome and Constantinople in the Fifth Century.
I would also learn about the Crusades, the Inquisitions and the Reformation of the Catholic Church.
As for the paintings, some were Byzantine (Greek) some were not. Many painters were commissioned by the Church.
Here's a bit of a timeline for you:
330 The city of Constantinople is founded by Emperor Constantine which he hopes will become the new capital of the Roman Empire instead of Rome.
380 Establishment of Roman Church with two popes, one in Rome and the other in Constantinople
451 Council of Chalcedon - Coptic, Syriac and Armenian Churches split from Catholic Church.
1054 Great Schism - Western Catholic Church (Roman Catholic) splits from Eastern Catholic Church (Eastern Orthodox). Western Church retains Latin in liturgy, Eastern Church in Greek.
1095 First Crusade launched by Pope Urban II at the request of the Byzantine Emperor to free Christian lands from Muslim invaders
1453 Siege of Constantinople. Byzantine Empire collapses and falls to Muslim Ottomans. Orthodox Church is split and Roman Catholic Church becomes most powerful Church in Europe.
1500s Two Protestant movements - England separates from Catholicism because of Henry VIII and Martin Luther creates separation in Germany