import arcpy
import os
import fnmatch
import glob

# getting list of mxds
if rec:
    # going recursive
    matches = []
    for root, dirnames, filenames in os.walk(root_fld):
        for filename in fnmatch.filter(filenames, '*.mxd'):
            matches.append(os.path.join(root, filename))
else:
    # going flat
    matches = glob.glob(os.path.join(root_fld, '*.mxd'))

# create thmb
for m in matches:
    mxd = arcpy.mapping.MapDocument(m)
    mxd.makeThumbnail()
    mxd.save()

...
if not mxd.hasThumbnail():
    mxd.makeThumbnail()
    mxd.save()

Public Sub test()
    Dim pMapDocument As IMapDocument
    Set pMapDocument = ThisDocument
    Dim pic As stdole.IPicture
    On Error GoTo eH
    Set pic = pMapDocument.Thumbnail
    Exit Sub
eH:
    MsgBox "Map Document has no thumbnail!", vbExclamation
End Sub