why are black people nearly always placed at the left or top left in
magazine photos, on tv screens, on posters, in movies, and in
advertisements?  please don't tell me it isn't true, as i have
observed this practice for years.

Prove it!

Get a set of dice. Use them to sellect a 1000 random Photos.

Rate each photo according to your theory. Publish your results.

Go on the talkshow circuit.

I don't think that it is true - but you may be onto something
- certainly I've never noticed it

However, from old brochure layout experience, I believe that usually
peoples' eyes are drawn to the right hand side of the right hand page
first.

eg: the 'value' of a spread increases from left to right
    (also from top to bottom)

Pictures are composed or selected to be eye catching, so, with a
couple, I would expect the woman to be on the right and the male on
the left, simply because both sexes tend to look at the woman first.

You might have a point, but it is probably a bit more complicated.
- have you tested your theory on pictures specifically targetted at minorities ?

First, it's true that before making such a categorical claim, you
ought to demonstrate that's it's true in some objective way. I'll give
you an example from my own experience.

   I tend to engage in a minor form of obsessive/compulsive behavior.
When driving my car I tend to reflexively "sum the cardinailities of
the digits" on the various car license plates I see. For example,
suppose I see a plate which says 67U4128. I  discard the letter 'U'
and do the following with the digits:

6 + 7 + 4 + 1 + 2+ 8 = 28

   Now, in the course of having done this steadily for nearly a
decade, I notice something specific; I'd swear that the digits sum to
the number 20 more often than any oher value. It seems to show up
clearly to me in greater relative abundance than other possible
numbers. But does it really? I don't truly know. There are at least
two possible hyptheses. One is that indeed the sums do come to 20 more
often than any other possibility. The other is that my brain has a
fondness for 20, and so I selectively notice it more than others.
Perhaps the distribution of license plate sums is rather flat. Or
maybe the distribution does have a camel's curve shape which peaks
around, shall we say, 36.

   How can I find out? I can collect data, and do a statistical
analysis upon it. I could for example go out on a busy interstate
highway and start recording license plates. Presumably an interstate
highway would have a good, unbiased (or at least negligibly biased)
selection of cars from a large area. Once the data are accumulated I
would then do all the sums, and then organise them into a histogram
according to the number of instances of each sum from a whole spectrum
of possible ones.

   But- I haven't collected the data; I haven't crunched the numbers.
So I've no objective basis on which to propose that the sums really do
tend toward the number 20.

   So as redfoxjumps recommends, you should do a study.

On the other hand, I wouldn't be a bit surprised if your observation
is spot-on, at least in relation to well controlled images such as in
advertisements & posters. It's no secret that marketing is a field of
research in which big business invests heavily. Marketing science has
itself been a big business at least since the 1940s. (Why do so many
automobiles have names which end with the vowel "a"? Precisely because
market research found that, at least in the United States, car buyers
respond to it favorably.) It's not out of the question to guess it's
been determined that, at least in some cultures, it's optimal to place
various ethnicities in certain visual arrangements.