I follow what sells in my portfolio. If a certain type of food is selling well, let's say cheese, then i do some research what the agencies already have and what is missing. Then i try to come up with something useful for customers.
In the beginning i also bought stock and used it for my work,manuals,flyers, tradeshows, print advertiding etc...if you work like a buyer you understand what kind of images are useful.
I also study any advertising i see for composition,colours,lighting,mood....to learn from it.
Any business or service i use, i ask myself what kind of images do they need, what will increase their business.
Stock is not about me. How can i support others in their work is what it is all about.