They're easy! If you're using a set of cube faces, or an equirectangular or cylindrical file, then SpinControl:VR can auto-generate a lo-res preview for you with one click.
QTVRs are the only exception: with them you will have to use your QTVR production software to include a preview in the .mov file -- this preview can then be used by PanoSalado, via its php QTVR parser script. SpinControl:VR will automatically include the necessary PHP parsers and files for you.