Do you or does anyone you know have pictures of loved ones around the house? Or have you been in a museum and seen paintings of people that used to be in their homes? Why? The picture is not the person, so why keep a picture of that person?
The answer is the same reason we have icons, images, and statues. The world was not blessed with photography at the time of Jesus, so paintings, icons, and statues will have to do. But they are there in order to see an image of our loved ones.
The Israelites made a statue of a cow and worshiped it as a god. They didn't make a statue of the God of Abraham who led them out of Egypt, no, they made it into a cow as a retaliation against God for leading them into the desert. They purposely made an idol into something other than God.
Now, back then, God didn't have a face... Jesus hadn't come yet, so how would they be able to even make a statue of God to begin with? But we do have an image of Christ now, and we have the image of Mary and the saints. We love them and just as we would have a picture of a loved one, we have pictures and images of them.
And to reiterate the far overdone accusation of Protestants, Catholics do not, I repeat do NOT worship Mary and the saints. We venerate them and ask for their intercession because they are already in heaven and can see God face to face. Do you not ask your friends to pray for you? Then why not ask your friends (or your grandmother) in heaven? Cannot they plead for you better than anyone here on earth?