Why are religious statues and sculptures often referred to as "images"?

Hmm. Well. Any visual art form is an image in a certain sense because it is viewable by the eye. As your other answerer says, they do also call sculptures "graven image" - however what you may be seeing could also refer to the actual image/photo of the art work. "Please view this image and write 500 words about it." This could refer to any type of art because it means look at whatever is in the picture. If I were you, and you're talking about a sculpture, I would use that! That will never let you down. If you're talking about a PHOTO of the sculpture, you can say "Image". :) If I saw something in person I doubt I would call it an image unless it was 2D. However remember that "image" can also be very broad (such as your "image" or the identity you show others). Depends! BTW this is not exclusive to religious "images" and the use of "image" has no direct ociation with religion.