An isolated object can appear to float when it has no contact shadow. A small oval or line shadow can ground it, provided the shape fits the object's weight and the scene's light.
Pick a shadow type
In the NexTools Shadow Generator, make a soft oval for a raised object or a tighter line shadow near a flat base. Put the PNG beneath the cutout. Align the darkest point close to where the object meets the surface, then reduce opacity as the shadow spreads.
Compare related assets
For a set of isolated products, use similar blur and light direction. A mug with a long hard shadow beside a bottle with a faint diffuse one will look as if the two images were shot under different conditions. Check each at thumbnail size, where grounding matters more than intricate detail.
The shadow should support the object rather than become a separate graphic. If you cannot identify the light source, simplify and soften it.
