Final answer:
To add a shadow to a picture, Karen should use a feature specifically meant for adding or manipulating shadows in photo editing or graphic design software, such as the "Drop Shadow" layer effect found in Adobe Photoshop.
Step-by-step explanation:
To add a shadow to a picture, Karen should select the option that allows her to modify light and shadow effects directly. Compressing the picture won't add shadows, using the Glow effect will add a halo-like effect around the image, and applying the Reflection effect will imitate a mirrored image, which does not include adding shadows. The correct approach for Karen to add a shadow to a picture would be to use a feature specifically meant for adding or manipulating shadows, often found in photo editing or graphic design software.
For example, in programs like Adobe Photoshop, there's a "Drop Shadow" layer effect that can be applied. This feature allows users to adjust various aspects such as opacity, angle, distance, and spread to create a realistic shadow that enhances the 3D appearance of the image. Other software might have similar functionality, so Karen should look for terms related to shadow in the software's effects or layer styles menus.