From a CGI point-of-view, it was probably the easiest image for them to use. *shrug*

As my understanding of the shawl goes, the image is actually supposed to be color, like a painting, but without all the brushstrokes and stuff.

This brings up a good line of conversation:

What editing and production mistakes did people notice in the movie?

One thing I noticed was that you can see Jim Cavaziel's real hand behind the fake one they actually nailed to the cross.