is it possible to tile backgrounds yourself, or does nsmbe have to do it for you?

NSMB Editor already tiles the background itself. Just click "Import PNG" and it'll tile your background.

If you want to manually tile a background yourself, the files are in the ROM data; the folder /BG_ncl contains the palette files, /BG_ncg contains the textures/tiles and /BG_nsc contains the background made of those tiles, and those are what you see in-game. You'll have to look at each one until you find the background you want to edit.
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well, actually i meant if you could choose which tile went where within the display grid of tiles to paint with.

okay, i'll do that.

you can just go the the background tab and double click on whichever background you want to edit instead of looking at the files.
even if you do look at the files though, just use this table to find the one you need. much quicker than checking everything https://nsmbhd.net/thread/3276-tilesets-and-backgrounds-name-table-guide/
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.