I have to agree with the general assumption over at Gallifrey Base that all those Doctor's weren't in the same room at the same time for that picture (indeed, I wouldn't rule out the possibility that Colin and Sylvester are sitting on the same chair...).
I mean, just look at the way Tom's standing. Either he's secretly operating a giant Peter Davison puppet, or he's been photoshopped in from a picture where his hand was resting on a wall.
Interestingly, on their forums Big Finish have apparently dodged the question of a photoshop job by just saying the picture of McGann in costume was taken especially rather than being reused from Dark Eyes
(due to his long standing problem- at least as far as Big Finish getting pictures of him goes- where he doesn't mind "In character" pictures but feels uncomfortable when photographed as "Himself", hence him being putting on the jacket).
That's not to be disparaging to the picture as it's a fun image, but it can't really be held up as an example of how easy it is to bring them all together. And for what it's worth, in The Light At The End
itself, Tom and Paul recorded together, Sylvester and Peter recorded together and Colin recorded by himself. So even if post production editing puts them all in the same scene Big Finish haven't actually managed to beat the TV version (with Ten and Eleven) in getting more than two of the actors together at the same time.
Indeed, I think The Sirens of Time
is the only BF story with more than two Doctors in it to have them all together in studio (Zagraus and The Four Doctors had all the parts for their brief reunion scenes recorded piecemeal).