An HD texture pack

Dump the textures the game draws, upscale or repaint them, and load them back as a pack. Packs replace textures at draw time, at any resolution, and can be switched while playing.

Dump the textures

In the game, open F9 › Textures and turn on Dump textures. Play through the areas you want: every texture the game draws is saved once as a PNG named by its hash in bin/texture_dump/.

One folder per level puts each level’s textures in its own folder (texture_dump/414 Zephyra Landing/).
Textures the game redraws on most frames (effects that change constantly) go to _dynamic/: a replacement could never match them, so you can skip that folder.
A mod’s own textures go to texture_dump/<mod id>/, each with its own _dynamic, so your pack of the game’s textures never ships a mod’s by accident.

Upscale or repaint

Run the PNGs through an upscaler, or paint over them. Keep each file’s name: the 16-digit hash at the start is how the game finds it (suffixes an upscaler adds after the hash are fine).

Make it a pack

Put the images in a folder under bin/texture_packs/<your pack>/. The port writes the rt64.json that lists them the first time you pick the folder.

Use it

Options › Textures, or F6 to cycle packs while playing.

As a folder or an .rtz (a zip with rt64.json at its root) for bin/texture_packs/, or inside a mod’s .nrm so it shows in the Mods list.
Higher in the Mods list wins between two packs; the pack picked in Options beats every pack in the Mods list.
A data mod that replaces a texture’s pixels changes its hash, so a pack made for the original no longer matches it.

Upscaled textures are still made from the game’s art. Share them only the way your community shares HD packs, and never include the ROM or other game files.