Are you asking for images or a complied package?
91x91? 50x50? Larger? Square? Round? Alpha channel (transparent background)? Top down?
Or just portrait sizes?
How many portraits would you consider a good pack?
I can't give a solid answer for the number of portraits since I don't know what the art costs on Mongoose's side are. Obviously, they'll only bother doing it if they can at least turn a modest profit on it. 12-15 male and 12-15 female human portraits and maybe 8-ish portraits for each of the common alien races would be nice. Obviously, the larger the numbers, the better. But that feels in line for the value of the $30 price point for other Mongoose products. Alternately, break it up into human, vargr, etc packs. The price point could be a bit lower but Mongoose could make some extra money from the completionists.
If we take the vargr pictures in the core 2022 book (page 54) as an example, just the headshots would be sufficient. It would be nice if we could get a waist-up or chest-up view but I have no idea how that affects the artwork cost. Extras like action poses, weapons, tools and backgrounds are actually negatives here since they limit the broad usefulness of the picture. For example, the two whole body shot vargr pictures would look odd if they're supposed to be a pair of high society merchants. The grey vargr could potentially work if sufficiently cropped down but the cutlass, pistol and eyepatch of the brown vargr pretty much limit her use to pirate, corsair, arms dealer or the like.
The key here is portraits that are usable in as many settings as possible. That might make them a little generic but I'd rather have 20 slightly generic portraits I can use in any context than 20 that are all very specific for the context you can use them in. E.g: A portrait of a woman that looks very upset and has a bunch of gang facial tattoos is hard to use as a cheerful baker NPC or stuffy corporate stooge even if that angry woman portrait has more character to it. Try to avoid showing too much of military or other uniforms that limits the use of a portrait to only, say the Zhodani navy but instead just show a military uniform-esque collar or shoulders and that portrait could be used for other militaries or militias, mercenary companies, corporate security, veterans, etc.
A transparent alpha channel for the background is the best since that maximizes the ways you can use it but a white background is broadly useful and can be removed pretty easily. Complex backgrounds should be avoided since they are, again, opinionated about how the portrait can be used and are a pain to remove.
Since these would often be used on a white page or pasted over another background, the shape isn't terribly important beyond fitting into a rectangular space reasonably well. (like a typical character sheet picture spot)
As for size, that's kind of irrelevant, IMO. I'm guessing that the artists are generally working with pretty high resolution when they are making them and it's in digital format, so the portrait size is easy to change. Just include a fairly high resolution for each (at least 1/4 page) and the user can scale it to use.
Given a choice between full-body pictures and just upper torso or head shots, the latter are better since that presumably means a larger number of them for the same art budget.
