Careful with those idioms!

While editing my historical novel, I realized once again how circumspect I have to be in my use of idioms. For example, I wanted my heroine to say something like, "I want him out of the picture", but could she have said something like that three thousand years ago?  In those days, folks didn't have pictures of other people, neither hanging on their walls nor tucked away in some album.  So this phrase would not have made any sense to her.
  Here is another one.  When faced with a difficult decision, could she have said that she wanted to "go over something in her head?" Given the fact that the brain was considered to be nothing more than some sort of filler material in those days, with no other function whatsoever, how could she have associated 'thinking' with 'head'?  Or when tempted to do something she knew was not wise to do, could she have wondered whether to follow her heart or her head?
  Well, I decided to give myself some artistic license, because if I followed the above reasoning all the way through, I might not have any English language left to work with. The 'out-of-the-picture idiom' is, well, out of the picture, of course, together with many other modern terms, but I can't reject everything that might not have been in the vernacular back then.
  One thing is for sure, though: I am spending a lot of time researching every phrase that crosses  my mind as I write. It slows me down, but will in the end have been well worth the effort.