A lot of users try to import some .png pictures (for example logos) to their CAD drawings. This can usually be done through some bitmap import function found in the respective CAD software.
Another possibility, albeit very strange one, would be to try to actually convert PNG image to a DWG drawing. This may be potentially useful for tracing or presentation, but 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.