So, the "billboard" image dump constitutes some 1200+ UI elements that mostly seem to be used for some sort of story and quest log feature.
Temp images are marked with the telltale red 仮 (kari, lit. "temp") stamp.
This includes large background and header images for each of the game's chapters and smaller screenshots representing each step of the main story, and header images for each subquest with smaller screenshots for each step of the subquests.
The chapter images seem to perfectly match up with the previously listed chapter titles, with 18 chapters in total.
Images are in various stages of completion which seem to indicate that Chapters 9, 10, 11, 14, and 15 were the least complete sections of the game when these images were taken, and feature unfinished versions of the Sector 5 Slums, Wall Market, portions of the Sector 6 Sewers, and the Sector 7 Plate. Everything else looks pretty much finished, apart from a couple Visual Works placeholders for pre-rendered cutscenes in the later portions of the game.
I may go into depth on how exactly the story breaks down and where certain setpieces appear later on, but suffice it to say that the entire game is laid bare here.
Notably, Wall Market seems to have been expanded considerably, and now resembles a sort of "Japan Town" (Wutai Town?) within Sector 6, and seemingly includes a large fighting arena. Other UI elements suggest that Sector 6 is the location of the Chocobo Stables, and that you can use a Chocobo Carriage to fast travel between different areas of the game at certain points.
It seems that the DeepGround enemies are encountered in Chapter 14, seemingly part of a subquest with a Shinra scientist that involves the Sector 5 Slums and the Sector 6 Sewers. The DeepGround Behemoth is a large Behemoth with glowing red seams/cracks all over its body, and the DeepGround Monster slightly resembles a humanoid monster, again with glowing red seams covering its body; it seems that it may come in a couple variants, and perhaps has some sort of additional appendage that resembles an insect arm growing from its back.
Interestingly there seems to be only one image of Roche and he's wearing his SOLDIER helm. It may be that his role was expanded later in development.
Other images are used for other menus, including the character menu, with renders for the playable characters, accessories, and weapons. Notably, Barret has renders both with and without his shades, suggesting that at some point in the story he loses or removes them.
There are images for an apparent character database, many of which are marked with the "temp" stamp.
Most of them are the usual suspects (party members + Turks + Shinra execs), however Kyrie Canaan, Leslie Kyle, and Mireille Dudley from The Kids Are Alright are all featured. Kyrie seems to be involved in quests concerning Shinra propaganda, and Leslie seems to be involved in some of the Sector 6 / Wall Market quests. Mireille is simply a placeholder with the text ミレイユ (Mireiyu) written out.
There are also two new characters, "Madam M," the geisha-style Madam of Honey Bee Inn, and "Andrea," a stylish tough guy who looks like he may be the main bouncer of the Honey Bee Inn. As mentioned before, Roche seems to be absent.
There are several images for the Jukebox, which includes album covers of real game soundtracks as well as fictional ones, which may be placeholders -- notably some album covers made from the various posters appearing in the game, such as for Loveless, Banora White apple juice, Shinra Oil, and Clear Icicle mineral water.
Most if not all the key items in the game are included, though many are placeholder images. Some of the names and images are interesting, though. There is a (temp) Shinra security card belonging to Rowan Rasberry, a "battle report" item that resembles a handheld scanner of some sort seemingly provided by that kid with the eyepiece scouter thing, arena coins and an arena ticket (signed by "M" with a kiss), placeholders for different tiaras and dresses, a "mog club" membership card which seems to be a kid's club, a Limit Break training manual for each of the four playable characters, and a Shinra "ruins" key card.
There are icons for "skill points" and "skill reset."
There are a few images that seem to be for a location database, similar to the character database. Again, most marked with "temp" stamps. Some interesting ones are "Hojo Institute" and "Secret Institute" which are clearly some sort of operating room with robotic surgical equipment.
There are a number of "Manual" images that showcase gameplay mechanics and controls. These include game features with names such as Battle Report, Wall Market Colosseum, Enemy Report, Game Clear, Jukebox, various minigames (box breaking, squats, dancing, etc.), Shinra Simulator, Subquests, Summon Coins, and VR Battles.
There are images for several of the "weapon abilities" and Limit Breaks in the game.
That's pretty much it.