Many users are trying to import some PNG pictures (for example logos) to their CAD drawings, which is what png to dwg conversion most likely represents. This can usually be done through some bitmap import function found in the respective CAD software.
Another possibility, although somewhat strange one, would be to try to actually convert PNG image to a DWG drawing. This might be for example useful for tracing or presentation, however the results are typically far from spectacular. Some graphics programs (especially those that deal with vector graphics) may be of help here, but the results may be poor.
Open PNG file Open DWG fileA 3D CAD translation software
A CAD CGM converter