I've read that the Eta Carinae becoming a supernova, exploding, whatever..... would affect us with highly nocive, potentially lethal effects, under 7500 years after the explosion/whatever really happens, because it's that far from us in light years.

Could be that Eta Carinae already exploded/became a supernova/whatever, and its effects are very very soon to reach us, I mean, not in thousands or hundreds of years, but in years, months, weeks, or even days?

As if it has exploded/became a supernova/whatever nearly 7500 years ago.

Or, by this 7500 years old image of Eta Carine we have today, scientists can say that it would last from there at least more 3000, 1000, 500 years?

(more or less like if I had a 20 years old photo of a man, who seems to be 40 years old in this picture, I can say that he's probably not dead, that he's only 60, assuming that the average lifespan where he lives is 80 years, and discarding that he died in any "non-natural" way)

Supernovae are rare, and not perfectly studied. At any rate, the consensus is that the star will go supernova in a few thousand years' time:

http://www.space.com/scienceastronomy/eta_car_031202.html
"It is more likely, however, that Eta Carinae will go supernova long before it simply withers away. That dramatic event could occur within 10,000 or perhaps 20,000 years"

But the star experienced an unexpected brightening in the 1998-99 period, and here's a man speculating with the possibility of that as a sign that the star has exploded yet
http://www.valdostamuseum.org/hamsmith/13Mar41.html#ecardetonation
Warning: the man speculating that possibility is Tony Smith, that has been banned from arxiv, so take his ideas with a pinch of salt


What will happen to Earth?
Again, there're differing points of view:

http://www.space.com/scienceastronomy/astronomy/milan_eta_carinae_000307.html
"Some have speculated that a huge blast of gamma rays could also affect the upper atmosphere, including the ozone layer. But that remains only speculation, and any such effect is likely to be very transient because the blast of gamma rays would be fairly brief.

The only humans who might suffer directly from Eta Carinaes violent demise would be astronauts in space. Outside of the Earths protective atmosphere they would be subject to the same powerful radiation as satellites"

Then again, this article by Alvaro de Rujula
http://arxiv.org/abs/astro-ph/0110162
threatens in the abstract that "At any time its core could collapse into a black hole, which may result in a gamma-ray burst (GRB) that can devastate life on Earth"

Gamma Ray Bursts caused by the collapse of hypergiants like Eta Carina are very directional in nature. It seems that two very powerful beams come out of such explosions, and if we were caught by such a beam we would experience some bad effects.
But many astronomers (including Rujula in the link given above) think that the star (or stars) in the centre of the Eta Carina nebula is oriented so that such a beam would miss the Earth.