Though I believe you can include cast photos if you host them locally (which I’d hate to have to do), or include links to them all at TMDB.
Infuse supports the .nfo <actor> tag, and additionally its sub-tags <name>, <role>, and <thumb> — but not <order> — as seen in this snippet from Firecore’s sample .nfo:
<actor>
<name>Gal Gadot</name>
<role>Diana Prince / Wonder Woman</role>
<thumb>https://image.tmdb.org/t/p/original/fysvehTvU6bE3JgxaOTRfvQJzJ4.jpg</thumb>
</actor>
I personally hate the Firecore added “support” for custom cast info to Infuse — it broke .nfo usage so badly, I can’t use it at all.
.nfo was once a perfectly convenient way for me to use Sort by Title to control Infuse’s sorting of my collection. I prefer the majority of my film franchises grouped together, in chronological order — but with all films remaining as individual movies in the Library and not hidden in a Collections folder). Previously I could edit titles to custom-sort a franchise like Indiana Jones and keep Raiders of the Lost Ark sorted ahead of the following films in the trilogy (or quadrilogy 
), and not orphaned alone in the Rs. Or, for another example, keep the entire Jurassic Park and Jurassic World series’ together without The Lost World (Jurassic Park 2) winding up orphaned in the Ls.
Rarely does a series’ chronological order match their alphabetical order (as the first three Bourne films did … until the 4th and 5th came out).
Further, I would use .nfo to customize the film’s Titles to distinguish between multiple versions … for example:
I’ve got a few versions of A New Hope: I’ve got the vandalized blu-ray Special Edition; and the far better fan reconstructions 4K77 and D+77. I’ve got 4 copies of each LoTR film; Theatrical and Extended each in both HD and 4K. What are there, maybe 5 versions of Blade Runner? 3 of Apocalypse Now? You get the Idea.
[ I don’t care to use Infuse’s new grouping feature because I invested a lot of energy into creating my own unique posters and fanart for each version and I prefer they all be displayed in their full glory independently. ]
Sorting by filename is workable enough alternative with movies (since Infuse actually sorts by folder name if there if only one film per folder — so the actual media files’ filenames can remain consistent with TMDB identification) — but since, (apparently unknown by Firecore until I brought it up), Infuse will not and, as structured, can not sort TV Series and TMDB Collections by filename, it was preferable to me to sort by title and update my movies’ titles via .nfo, to compensate for being unable to use filenames throughout the app. HOPEFULLY Infuse will someday give us the option to sort different categories and views and folders each by our own preference instead of one global preference; but until that day, or until Infuse fixes the cast and crew search and .nfo issues; I’m stuck having to switch between the two views and my experience using Infuse since this happened is definitely worse than it was when I began using it — (that is, apart from when Firecore finally fixed my original day-one pet-peeve — the unnecessary splitting of TMDB’s three ampersanded TV genres).
The root of the problem is that if you include Cast and Crew tags in your .nfo — and more insidiously, even if you REMOVE cast and crew TAGS from your .nfo — the very presence of an .nfo causes Infuse to disconnect your title from it’s TMDB-derived cast and crew database.
Even if you have no cast & crew tags in your .nfo — as I didn’t, because I didn’t care to change the info on TMDB, which is certainly accurate enough for me — suddenly every movie and TV show has ZERO cast members visible on the titles’ details pages.
What’s really happening here is not what one would desire — that IF cast tags exist, they be incorporated into Infuse’s existing cast and crew database.
Even IF EVERY SINGLE .nfo FILE for EVERY ITEM IN YOUR COLLECTION includes the full cast and crew details downloaded from TMDB itself (via Kodi or TMM) — and thus is identical to Infuse’s own — Infuse STILL CREATES A SEPARATE DATABASE. And only Infuse’s now mostly invisible TMDB-sourced database is build and completed in the background by unobtrusively and completely indexing one’s entire media collection.
So no matter what, Infuse still downloads cast and crew metadata and images from TMDB, and uses that in its primary database. Only, Infuse won’t display that information to users who have .nfo files. Instead, it only displays the cast and crew members specifically included in the .nfo — for which Infuse CREATES AN ENTIRELY DIFFERENT DATABASE — and only if it their titles were indexed.
THESE TWO DATABASES ARE SEPARATE AND INCOMPATIBLE.
If you select an actor’s headshot (or their generic icon when images haven’t been imported), the only results you’ll see are that actor’s films or series which you’ve already accessed by navigating to them in the UI — not results from all the titles extant in your library which they actually appeared in.
Unlike with Infuse’s own TMDB-sourced cast and crew database, the .nfo-sourced database is not created in the background. Cast and crew members’ films and TV appearances are only added to the database “on demand” — that is, only after you, the users, browse to (and perhaps, necessarily) open up a given title that they appear in.
Worse, if you conduct a manual search for an actor via the Home Screen search 
button, you’ll get TWO results for every actor who was scanned into the .nfo database. That means, two identical headshots, side by side.
If you click one, you’ll be shown their complete filmography. If you click the other, you’ll only see a very limited list; because you wouldn’t have navigated yet to view every title in your collection.
There’s no way to tell the two headshots apart.
Firecore has broken cast and crew search completely.
I’ve wrote extensively about this to @james — but it seems not to be a concern of theirs at all.
This “new feature”, I imagine, isn’t really benefiting anyone; and it has RUINED a huge part of the Infuse experience for me. This “feature” has been the largest turd Firecore has yet dropped since I began using and loving this software.
It boggles my mind how it has gone this long without correction (especially considering that searching for cast and crew through the icon was further broken a few updates later).
Since Infuse users cannot create custom movies or tv shows with .nfo files from scratch — I can’t see why their would currently be any demand at all for allowing custom cast.
If a cast member is missing from TMDB’s cast database, one can far more easily go to TMDB and add them. Doing so will help not just one’s self but all fellow metadata consumers.
So, yeah, what you’re seeing is the currently expected behavior of Infuse; because Infuse is broken.