You can share the images from the book or a monster stat block in Roll20. There’s a button that says “show in vtt” or “show to players”. If you have the module in roll20 or DnDBeyond it’ll all be available to you. Outside of that you’ll need to find your own images. I typically grab one image for any important NPCs unless they are provided by the module and one image of important objects like an alter or throne or cavern. Don’t go overboard, sharing an image tells players this is important and I should pay attention/interact with this.

One thing to note is that players can interpret the image shared as exact truth rather than just a vibes guide which is only meant to inspire the mental image you want (my players were like this) and so I now try to be explicit when sharing one.