diff -ru a/agb-image-converter/src/config.rs b/agb-image-converter/src/config.rs
--- a/agb-image-converter/src/config.rs
+++ b/agb-image-converter/src/config.rs
@@ -1,9 +1,9 @@
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 
 use crate::{Colour, Colours};
 
 pub(crate) trait Config {
-    fn images(&self) -> HashMap<String, &dyn Image>;
+    fn images(&self) -> BTreeMap<String, &dyn Image>;
     fn transparent_colour(&self) -> Option<Colour>;
 }
 
diff -ru a/agb-image-converter/src/lib.rs b/agb-image-converter/src/lib.rs
--- a/agb-image-converter/src/lib.rs
+++ b/agb-image-converter/src/lib.rs
@@ -6,7 +6,7 @@
 use syn::{Expr, ExprLit, Lit, Token};
 use syn::{parse_macro_input, punctuated::Punctuated};
 
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 use std::iter;
 use std::{path::Path, str};
 
@@ -149,7 +149,7 @@
 }
 
 impl config::Config for IncludeBackgroundGfxInput {
-    fn images(&self) -> HashMap<String, &dyn config::Image> {
+    fn images(&self) -> BTreeMap<String, &dyn config::Image> {
         self.background_gfx_options
             .iter()
             .map(|options| (options.module_name.clone(), options as &dyn config::Image))
diff -ru a/agb-image-converter/src/palette16.rs b/agb-image-converter/src/palette16.rs
--- a/agb-image-converter/src/palette16.rs
+++ b/agb-image-converter/src/palette16.rs
@@ -169,7 +169,11 @@
         let optimised_palettes = packed_palettes
             .iter()
             .map(|packed_palette| {
-                let colours = packed_palette.unique_symbols(&palettes_to_optimise);
+                let mut colours: Vec<_> = packed_palette
+                    .unique_symbols(&palettes_to_optimise)
+                    .into_iter()
+                    .collect();
+                colours.sort();
                 Palette16::from(colours).with_transparent(transparent_colour)
             })
             .collect::<Vec<_>>();