I think that artists often included pagan images in their art in order to grab the attention of the people who had these beliefs. These images were something that these people were familiar with. They felt more comfortable in the company of these images.
Christians wanted to get their messages out and expose more people to their beliefs. Because of limited literacy, art was a very important vessel in which to communicate their message. By nature, many people will only take the time to look at or read something that catches their interest. Many times, we tend to skip over anything that is new, different or makes us feel uncomfortable. By combining both pagan and Christian elements in art, the attention of the audience in caught and combining both pagan and Christian elements in art also conveys the message.
If the pagan people have beliefs that were based on “magic” and the desired results were not always obtained, it was hard to understand. With the Christian faith, there was always the faith in a higher being that knew best. Even though people still could not control the final outcome, they were assured that things work out the way they do for a reason and in the end things will be better. Of course, the Christian people had to convince them that this was the case. Having both pagan and Christian images in a work of art could help convince people to accept Christianity.
Pagans were accustomed to placing their faith where they could not control or see the final outcome. It was not a far stretch to think that they would easily transition their faith to Christianity, where the final outcome was always good if not in this life, but in the life after.