Just like all the other aspects related with your brand, your imagery changes constantly over time as trends and technologies evolve, people's desires and habits change, and new products and services enter the market.
It's important to establish a standard for consistency and brand image at this stage so you can maintain them later. For example, do want to adopt a visual style or do you prefer to use photography?
If you want to include illustrations, specify which kind of illustration best fits your target audience and aligns with the brand identity. For example, if you're creating an infographic for a business blog, then you might choose a 3D illustration. If you're going to be taking photographs, are you okay using pre-made stock images or would you prefer to take original photos exclusively?
It's important to specify your brand's imagery so that your team knows exactly what to expect when they're working on projects. It will act as an inspirational guide for them when they're looking for images or directions for UX/UI design.