http://www.marypages.com/Knock.htmIt's big but it may be the one you think is too old fashioned. The apparition was in 1879 but I'd say that image is post-1900. Have you found any that were drawn near the time of the apparition?
There's also a big photo of the statue of Our Lady of Knock if you haven't seen sized "Large." You have to scroll down to see it.
I don't know if it was a real apparition. I've read some claims that it was a hoax designed to draw tourists, which it has, but the Church investigated and approved it. I suppose a magic lantern could use candles or an oil lamp but don't really know. I wouldn't think they were lax in investigating back then or would have been ignorant about methods of producing illusions. It was certainly a lot harder to be canonized then than during JP II's papacy so it would seem that investigations of apparitions would have been equally rigorous.
If it seems real to you and something about it speaks to you in a spiritually enriching way, I think you should accept it as real. Everything is based on faith anyway, right?
Jesus gave us the Blessed Mother to be our mother so I think we should honor her, and choosing an image or images to visualize the BVM is an individual thing. I never consciously thought of this before but clearly it's what I've done. Our Lady of Perpetual Help and Our Lady of Guadalupe are my favorite images of Mary, I think because they're more iconic than many other images. I have favorite images of Jesus, too, of course.
Since Mary was human and God the Son took on human form as Jesus, we can visualize them a lot easier than God the Father or God the Holy Spirit. Thinking of the latter, I can't visualize images much different than the image of God creating Adam from the Sistine Chapel ceiling and the image of the Holy Spirit as a white dove, specifically the one in the round window in St. Peter's Basilica. How do you visualize pure spirit? Is it possible to visualize pure spirit?