Dear Jim,

It is interesting that two contexts of 'nimitta'

>> Kaara.na, Cause.
>> **************
>> ``adhicittamanuyuttena, bhikkhave, bhikkhunaa pa~nca nimittaani

JA> kaalena

>> kaala.m
>> manasikaatabbaanii''ti (ma. ni. 1.216) ettha kaara.nassa.

>> Vipassanaa.
>> **********
>> ``ya.m nimitta.m aagamma ya.m nimitta.m manasikaroto anantaraa
>> aasavaana.m khayo hotii''ti (a0 ni0 2.6.27) ettha vipassanaaya.

are the same, both interweave together in the Vitakka-santhana sutta
(MN 1.119).
http://www.accesstoinsight.org/canon/sutta/majjhima/mn020.html

The 'subha-nimitta' also belongs here, as
'that-which-when-attended-to-leads-to-change-of-mental qualities', as
in Ahara sutta
http://www.accesstoinsight.org/canon/sutta/samyutta/sn46-051.html
or Samvara sutta (AN 2.16 (4.14))
http://www.accesstoinsight.org/lib/modern/thanissaro/wings/2c.html#passages
where various contexts of 'nimitta' meet together.

Thus 'nimitta' is closely linked to the 'yoniso manasikaara' or
'fundamental attention', as it is also shown in
Niivara.nappahaanavaggo (AN 1.3)
http://www.chibs.edu.tw/exchange/CONFERENCE/4cicob/fulltext/PHRA.htm
Fundamental attention is that which is directed to certain classes of
'nimitta'.

Sometimes in the suttas 'nimitta' means not just 'perceptual image',
but some predisposition of apperception (sa~n~naa), attunement of
apperception to some perceptual images, as in Suda sutta
http://www.accesstoinsight.org/canon/sutta/samyutta/sn47-008.html

and such skillful attunement is a key to success in meditation:
http://metta.lk/tipitaka/2Sutta-Pitaka/2Majjhima-Nikaya/Majjhima3/128-upakkilesa-e.htm
http://www.accesstoinsight.org/canon/sutta/anguttara/an09-035.html

There's a colorful story in Visuddhimagga about Mahatissa Thera
http://web.ukonline.co.uk/buddhism/sakka2.htm#story
who was so well attuned to the apperception of bones that he perceived
a beautiful woman as a moving heap of them.

Best wishes,
Dimitry