I am also in total agreement about using actual 3D forms to "train the
eye" in drawing, but allowing students to shoot their own images for
certain projects seems realistic and appropriate. I am a former
commercial artist turned educator, married to an art director/fine
artist, and can honestly say working artists appropriate imagery all
the time, but they interpret the images and make significant creative
changes (or you can definitely run afoul of copyright laws). So from a
real-world standpoint, the 2D to 2D and 3D to 2D approaches both
should be covered....Additionally, with younger students (grades 1-5),
they work much better from existing pictures, but you still need to
model "creative interpretation" and talk about copyright laws.