Apart from good lenses, lighting is one of the essentials for technically successful photos. That’s why there are so many books on different kinds of lighting. I’d suggest you read some of them and never stop thinking about a good or even better lighting.
I couldn’t possibly cover every single aspect of the topic “good lighting” in this book. But I can still give you a brief overview and try to evaluate the importance of good light for stock photography.
Not only once, I’ve heard from a photo beginner: “I thought I need a flash in low light only.” But that’s just one side of it. A flash can be of a great help to lighten the shadows in a model’s face in the blazing midday sun.
Always bear in mind: The term photography ...