It is unlikely direct png to sh3d conversion is possible on file per file basis. What is however possible, is to PNG pictures as graphics assets in the Sweet Home 3D from which the .sh3d files come from in fist place.
PNG images can be probably used in the texture editor and saved as part of ,sh3t files (texture libraries). But .png files may also be used to import home blueprints and background images.
Updated: January 4, 2024