they always clearly differentiate between the millions/billions of years before creation and the "days" of creation.
You might well be right. It could be that there are some old-timers at HQ who won't let the 7,000-year thing go, or that they internally see some reason why it needs to be true in order to support their beliefs. It's probably just tied to their intense desire to see the end come in their lives.
Personally I don't know where the 7,000 years came from. We had a discussion on this a few months ago that basically concluded "*shrug*". This old, more careful discussion also concluded "*shrug*". I suppose it was just too convenient for people who wanted to believe they were in the end times, as you said. This isn't just a notion of the Witnesses; this specific formulation of "six thousand years and then the Thousand Year Reign" probably started in the Second Adventist movement. In the 1800s, Russell taught that the 6,000 years ended in 1872, I believe. In the second link I just provided, interestingly enough, it seems that people were suggesting "six thousand years of man's existence" 2,000 years ago!
I feel like that ought to be allowed, even if it isn't technically…
Of course people post the Society's stuff here all the time, so you may feel that there is precedent. I think one is legally supposed to be able to post excerpts from their writings, so maybe video stills will be allowed under that same principle. But lawful or not, they can probably issue a takedown notice on the basis of copyright, so one day you may find that your host has shut down your site. You might be able to get it back up again by simply removing those images, but I just thought I would give you fair warning.