In most cases, the actor is ignored if it already exists although there are a couple of exceptions.
eg, movies, since the actor image always comes from IMDB, MC will not overwrite it.
For TV Shows it does overwrite it, since the actor could be coming from TVDB or IMDB, depending on settings, and there is no certainty that the user has not switched preferences.
As for the different types of actor images, what you suggest is not supported by XBMC, i've gone as far as their specification allows.
I suppose you could add different images to the various folders,
as long as your tv folder structure is done by seasons then you could add actor images for each season,
'The X-Files\Season 01\.actors\'
If you really wanted, you could even have seperate actor images for each episode, by having each episode within its own folder
'The X-Files\Season 01\Episode 01\.actors\'
but where would you get them from. There are various sources for actor images, but I can't think of anywhere that gives a different image depending on the series number etc.