My storyboarding co-workers and I were talking just the other day about the days of old when illustrators and artists had to hoard huge filing cabinets filled with images clipped from magazines and books, photographs etc for the need of reference... How demanding that was on the time and space of an artist, not to mention the anxious sense that almost any new image might be worth capturing for the library and that you were constantly on the prowl. A few years ago one of the studios I was at inherited a legendary film artist's files and they were vast, requiring months of shipping, storing and organizing boxes and cabinets filled with photos and slides, enough to fill an the equivalent of a one bedroom apartment.
Now we have literally a world of reference merely a click away. A few more clicks and you can save hundreds of such images to a hard drive as small as a keychain. The only downside is that there are no real excuses anymore for not being able to make a viable sketch of whatever assignment you've been given. The only other caveat is that if you can get firsthand experience of something you have to draw, there still really is no substitute for that.
Here are two image examples that popped up on Facebook recently.
This image of Hitchcock pointing directly on axis at the camera is a beautifully taken shot. The finger point on axis is one of the hardest things to draw, in my opinion, especially since the key element of the pointing index finger is by necessity not in clear silhouette. The fingers obscure the hand proper, except for the oblique area in the web of the thumb and forefinger... It's a toughie, but this excellent photo helps... (although the shadow cast by the finger muddies the image a bit)
Granted, this is the sort of reference you could readily use a mirror for, but the're both keepers for me. Thanks internet, don't ever change...