You cannot exactly convert a 2D raster image (for example .jpg pictures) to a 3D objects (.obj files). However, some tools allows users to apply the picture as a texture to a 3D mesh represented by the .obj file. Typically done with a 3D modeling tool or editor, many of them natively support jpg to obj conversion.
Open JPG file Open OBJ filePMD, PMX, X MikuMikuDance model editor
A 3D CAD translation software
A texture mapping utility