The answer to your first question is that all the Catholics in the world go to one huge church the size of New York (up the street from the Anglican Church) called the Catholic Church, every Sunday. ;)
The second question is a trickier one to answer, so bear with me as I might take a little while to explain it. :)
We pray to saints in a different way than we pray to God. When we pray to God we know that our prayers are answered through his own power. When we pray to the angels/saints, we ask them to intercede between God and us on our behalf. Praying to the saints is like asking someone to pray for you. We are not praying in the sense that we are worshiping them or expect them to use their power to answer our prayers, because they cannot answer our prayers except with God's power working through them. So when we pray to the saints and angels we are not asking them to help us or thanking them for help received, but rather asking and thanking God through them.
You might ask what the point of that is, as it isn't as if the saints help with the backlog so that God can catch up later, and he can hear our prayers anytime. The reason for that is because the more prayers there are on our behalf, the more likely God is to answer our prayers. Again, this is not because God is hard of hearing and might miss us the first time, but because we believe in what is called the "Communion of Saints" which includes the saints in heaven, those in purgatory, and those on earth who are still working towards heaven. Communion means "union" and is related to the word community and that is sort of what the Communion of Saints is. Because all these people are in communion with each other (not meaning that we are all telepathically communicating with each other or are some kind of hive collective or anything like that ;) ), we pray for each other. (Praying for others is beneficial to those on earth because we believe that those on earth and in purgatory receive graces for prayer.) We believe the prayers of the holy (the saints) have more effect (James 5:16–18 gives the example of Elijah) than the prayers of other people. If you think about it, that makes sense because you are more likely to help someone who cares about you than an enemy. Of course God is not human and we believe he is infinitely merciful, but I don't want to get to deep into things right now so I will just leave it at "Scripture says that the prayers of the righteous are more powerful than the prayers of sinners" for now.
As Catholics we are encouraged to pray to God as well, and often, but since there is nothing wrong with asking the saints to pray for us, we do so.
Regarding veneration of images, we do not worship the images or the people of which the images are representative, but honour them as being examples which we should strive to imitate. When we make a shrine to venerate a saint and place flowers and other objects at the shrine, it is equivalent to placing flowers on the grave of someone who has died, in respect to their memory, but you wouldn't say we worship that person. If you lose a loved member of your family, you may have a picture of them in a prominent place to show your respect for that person, but you wouldn't say that it is idolatry. It is the same way with the saints. Because they were so close to God on earth we venerate them in a special way, so it is just a glorified version of what people do for deceased loved ones.
It can become idolatry though if you pray to a saint with the expectation that they will answer your prayers personally, or if you treat a statue or image as if it were alive and able to answer your prayers (i.e. if you pray to the statue and not the saint). When we honour an image we are just showing respect to that person, but should in no way think of the image as having power of its own.
Hopefully that makes sense and I didn't reiterate too much, as I am known to do that. :P