gfx4snes needs indexed images, but artists work in RGB. img2snes is the bridge: it quantises a full-colour PNG down to a SNES-legal indexed palette, so your artwork feeds the rest of the pipeline without hand-editing palettes in an image editor. Run it once on new art, commit the indexed result, and gfx4snes takes it from there.

What goes in, what comes out

In: a .png — RGB, RGBA, or already indexed.

Out: a palette-indexed .png (2–256 colours, palette embedded). The default name is <input>_indexed.png.

The flags you will actually use

Flag	Meaning
-i FILE	input image (required)
-o FILE	output path (default <input>_indexed.png)
-c N	number of colours, 2–256 (16 for 4bpp, 4 for 2bpp, 256 for 8bpp)
--round-snes	snap the palette to SNES BGR555 (multiples of 8) — recommended
-p FILE	match pixels to an existing PNG's palette (shared palette across sprites)
-s FACTOR	nearest-neighbour scale
-t N	pad dimensions up to a multiple of N (e.g. 8)
--batch	treat the input as a glob and convert many files

Worked examples

# Quantise a character sprite to 16 colours, SNES-rounded:
img2snes -i character.png -c 16 --round-snes
 
# A 4-colour (2bpp) font source, explicit output name:
img2snes -i font.png -c 4 -o font_4col.png
 
# Force several sprites to share one palette (pass a reference PNG):
img2snes -i enemy.png -p hero_indexed.png

Then hand the indexed PNG to gfx4snes — images to tiles, palettes & maps with a matching -u (16 colours → -u 16, etc.).

Gotchas worth knowing up front

Transparency = index 0. Pixels with alpha below 128 map to colour 0, which the SNES treats as transparent — so paint your background in fully-transparent pixels, not a "magic" colour.
--round-snes early. The SNES only has 15-bit colour; rounding at quantisation time means what you see is what the hardware shows, with no surprise shifts later.
-p for shared palettes. When several sprites must live in one 16-colour sub-palette (the Planning your SNES game constraint), quantise the first, then map the rest onto its palette with -p.
Optional and standalone. img2snes is not wired into the build — it is a one-time artist pre-step. Its output is what you commit and reference.

Where it fits

img2snes is the very first box in the The OpenSNES toolbox pipeline: RGB art → indexed PNG → gfx4snes — images to tiles, palettes & maps → tiles/palette/map. If your source art is already indexed (pixel-art tools often export that way), you can skip it entirely.