i would like to know how to convert a jpg/png to a mif file
In the coming MRtrix3 update, PNG images will be supported as a native image format. When combined with multi-file numbered image support, and the new command mrcolour, this enables all sorts of trickery. 
i created a 3d object of a face and would like to import it in MrView and/or overlap it with MRI data.
Your description isnât adequate to answer this question. The answer depends entirely on the fundamental nature of what you are referring to as âa 3d objectâ. My suspicion is that you are referring to 3D triangulated surface information, to which the answer is: we donât yet have the capability to display such information natively. The best we could offer would be to run the data through the mesh2voxel command, and visualize the output of that.