Hard Won Lessons
I spent almost a million dollars of OPM (Other People's Money) learning these five lessons about images and web design, so lessons learned the hard way:

1. Portraits Are Powerful
Portrait images where the model looks directly at the camera, are powerful "welcoming" images great for home, about and category "splash" pages.

2. Babies are DYNAMITE - Use Carefully
Thanks to our ancient caveman brain we can't NOT look at babies. Problem is that is not a secret so babies are now overused to hock insurance, tires and shampoo. If you use a baby my preference is to have the baby looking AT something.

Visitors eyes go where the eyes of people (or babies) are looking, so point your baby image directly at an important Call-to-Action and bet your conversions go up.

3. People Talking To Each Other = DANGEROUS
There may be context where it makes sense for you to have an image where people in the image are huddled together, but I doubt it. If you have two people huddled and a third looking directly out at the camera the image works better.

We respect a huddle. We don't want to intrude, so your web image is working against your online marketing purpose. Your image says we are here having a conversation and YOU (visitor) aren't invited. Not a good idea.

4. People Sell Better Than Widgets, but...
I  prefer to tell human stories even about the most widgety widget, but people bring "like me" problems too. Every visitor is looking for "like me" signals. If you know your archetype and tribe well enough to risk it use images of people consistent with your understanding.

If you have a wide variety of customers and members best to avoid single archetype "like me" images. This is yet another reason I like portraits. Portraits are "universal" meaning the welcoming look directly at the camera removes some of the "must be like me to engage" requirements.

5. In Action Shots Use The MOVEMENT
If your image is riding a bicycle POINT the movement at something important. I don't like movement images as heroes (largest images on a page is called a hero), but I love them in "sub-hero" images because movement creates excitement and allows me to direct the visitor's eyes where we want them to go.