I meant to get into all this on Saturday - but IE decided to have a problem with the BBC/Smileys buttons, so my explanation was delayed until today.
As Gothick has already surmised, the original series background is made up of captures taken from eps that originally ran in January. However, all the backgrounds are January themed in one way or another. The original movies background is made up of January captures taken from the Movies Quotes Slideshow, the '91 series background is made up of January captures taken from the '91 DS Series Slideshow, and the original series background is specifically made up of captures taken from both the 1990 Calendar Of Quotes Slideshow and what I like to think of as the This Date In DS History Slideshow (though we've never called it that). And, yes - each month there will be a new background in each category. But for now, here are the three current backgrounds:
The original series:
The original movies (currently featuring hods - will switch to NoDS in July):
And the '91 series:
And along with the backgrounds comes forum logos for each:
The original series:
The original movies:
And the '91 series:
And as was always the case with our previous theme, if your browser isn't showing these new backgrounds and logos, then you need to clear your browser's cache.
I'm also working on a fourth set. And if it turns out as well as I'm hoping, it will probably debut soon...