You can add "click here" or "buy now" wording to your image in Book Brush however it is not possible to add a hyperlink to an image to make a .png or .jpg universally clickable.
Making the image clickable is something that is done at the end within the program where you are placing the image. For example, making an image clickable on Facebook would be done on Facebook, not within Book Brush.
If you need help with this, you can try a Google search for “How to make my image clickable on [x]” to watch YouTube videos/tutorials to help!