Pretty much any raster images (for example PNG graphics) images can be used as textures for 3D objects (.obj files for example). This is not actually png to obj conversion per se, but more of a workflow to achieve this. Such function should be available in most 3d modeling tools, considering .obj files supported in almost everything.
Open PNG file Open OBJ filePMD, PMX, X MikuMikuDance model editor
A 3D CAD translation software
A texture mapping utility