Pick preview instead of main image

[DOS386]

Idea: Add an option to NC and XN to pick the stored preview image instead of the main one (JPG, DICOM, ...).

See also:

http://newsgroup.xnview.com/viewtopic.php?t=28547 "DICOM BUG ($88,$0200) preview"
http://newsgroup.xnview.com/viewtopic.php?t=28339 "Why is "-high_res" needed for RAW?"

This won't work with BMP GIF PNG ...