I suspect it depends on how 'featured' the person is.... but is also why the WT uses a large variety of different people, rather than using the same ones.
I'd presume the basic pool would be bethelites, appointed brothers with wifes & regular pioneers, within congregation meeting catchment area of each bethel/branch office
This would probably give a reasonable number - especially when you consider the number of offices around the world, even if they are reducing now.
I suspect that with the monthly broadcasts, they've needed to broaden out a bit
But as ToesUp says, it would be 'exemplary' publishers - so that's people who would be qualified to auxiliary pioneer/be used on platform at Regional Conventions - though I'd expect it would be more akin to the qualifications for being a Regular Pioneer - ie: not reinstated within the last five years, not reproved by a judicial committee for wrongdoing within the last three years.
For general background or groups of people (ie those making up the congregation in a KH scene,or street scene) they're probably not so strict - say just 'publishers' in good standing.
FWIW Barbara Anderson and her husband have been included on TWO FRONT COVERS -
Awake 8 October 1986
Awake 22 July 1992 (4th from left)