My question is about the lava object animation. (obj 19 in Castle tileset)
Does anyone know where in the ROM File Browser the animation of this is?

Nowhere. It's done by code. It uses the pallete of the tileset itself.

Custom animations like that one are already possible with the ASM hack  Dirbaio wrote for the Comunity Remix. He explained me how it works yesterday on IRC. I only got one problem I have to solve, then I'll explain it in the ASM forum
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Huh? I don't get it. I thought it would be just two images, like with the quicksand animation.

No it isn't. The game draws the lava tiles each frame itself.
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Okay, but.. is it possible to edit the graphics of the lava then?

I don't have time to check right now but I remember it being 3D textures in the polygon_unit folder (?)

You mean the Lava Tiles in Castle Tileset? If yes, I tell you that you can change it. The files are in the folder and BG_ncg BG_ncl, but I do not remember the name of the file.

Edit: In my hack NSMB New Adventure I tried and it worked.

I realy don't think that that graphics are in the polygon folder (-> 3D textures). A bit ago I made a tileset and I forgot about the animated tiles. So I got the old lava with the new pallete. Also the tiles are behind Mario.
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I found it!
It's in the polygon_unit folder (thanks  Dirbaio), and the file name is I_yougan.nsbtx.

But there's only one image, while it should be at least two, since it's an animation. I guess this is what RicBent was talking about(?)

EDIT: I just found out that it isn't I_yougan.nsbtx,
since when I edit the texture, nothing changes in-game.

The frames for the tiles used in the tower tileset is in d_2d_TEN_I_toride_ncg.bin in BG_ncg and the palette is in d_2d_I_M_tikei_toride_ncl.bin in BG_ncl. And for the castle tileset, the graphics are in d_2d_TEN_I_yakata_ncg.bin in BG_ncg, and the palette is in d_2d_I_M_tikei_yakata_ncl.bin in BG_ncl.
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Wow. Never knew that

I always thought that these were uneditable.
This is exactly how  Dirbaio's ASM hack works.
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Cool! Thanks  Arctus Platner
(And also thanks to the others who helped finding this out of course)

The conveyors are also in that image.
And I guess the rest of the image contains the animation of the flames of the tower/castle foreground?

EDIT: There's also the file 'd_2d_TEN_I_yakata_nise_ncg', which also contains the lava. What's the meaning of that file then?

As far as I can tell, it goes unused, since almost all of the castles use the same tileset, excluding W8-Castle, and W8-Final Castle, which use static tiles in the level for the lava tiles rather than the animated tiles used in the other castles.
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"Never use a flash drive unless you are transferring files." - Me, around 10 years ago