diff --git a/dither-image.js b/dither-image.js
index 88bbcbb..e502cfe 100644
--- a/dither-image.js
+++ b/dither-image.js
@@ -529,8 +529,15 @@ function decimate(input, palette, n, inputError) {
  */
 async function loadImage(src) {
     let image = await Jimp.read(src);
+
     let width = image.bitmap.width;
     let height = image.bitmap.height;
+    if (width != 160 || height != 160) {
+        width = 160;
+        height = 160;
+        image = image.resize(width, height);
+    }
+
     let rgba = image.bitmap.data.slice();
     return {
         width,
@@ -539,23 +546,6 @@ async function loadImage(src) {
     };
 }
 
-/**
- * Halve the horizontal resolution, interpolating input.
- * @param {Uint8Array} rgba
- * @returns {Uint8Array}
- **/
-function halveImage(rgba) {
-    let out = new Uint8Array(rgba.length >> 1);
-    for (let i = 0; i < rgba.length; i += 8) {
-        let j = i >> 1;
-        out[j + 0] = (rgba[i + 0] + rgba[i + 4] + 1) >> 1;
-        out[j + 1] = (rgba[i + 1] + rgba[i + 5] + 1) >> 1;
-        out[j + 2] = (rgba[i + 2] + rgba[i + 6] + 1) >> 1;
-        out[j + 3] = (rgba[i + 3] + rgba[i + 7] + 1) >> 1;
-    }
-    return out;
-}
-
 function imageToLinearRGB(rgba) {
     let input = [];
     for (let i = 0; i < rgba.length; i += 4) {
@@ -583,18 +573,13 @@ async function convert(source, nbits) {
         rgba
     } = await loadImage(source);
 
-    if (width == 320) {
-        width = 160;
-        rgba = halveImage(rgba);
-    }
-
     if (width !== 160) {
         throw new Error(`expected 160px-compatible width, got ${width} pixels`);
     }
 
-    if (height !== 192) {
+    if (height !== 160) {
         // @fixme support up to 240px
-        throw new Error(`expected 192px height, got ${height} pixels`);
+        throw new Error(`expected 160px height, got ${height} pixels`);
     }
 
     if (rgba.length != width * 4 * height) {
diff --git a/dither4.s b/dither4.s
index eba84eb..3f1e314 100644
--- a/dither4.s
+++ b/dither4.s
@@ -27,11 +27,12 @@ sample_ptrh = $85
 sample_ptr = sample_ptrl
 scanline = $86
 
-height = 192
+height = 160
 bytes_per_line = 40
 pages_per_frame = 32
 lines_per_frame = 262
-scanline_offset = 31
+;scanline_offset = 31 + (40 - 24) / 2
+scanline_offset = 47
 scanline_max = (lines_per_frame - scanline_offset) / 2
 
 .data
@@ -44,12 +45,12 @@ scanline_max = (lines_per_frame - scanline_offset) / 2
 .import audio_samples_end
 
 displaylist:
-    ; 24 lines overscan
-    .repeat 3
+    ; 40 lines overscan
+    .repeat 5
         .byte $70 ; 8 blank lines
     .endrep
 
-    ; 192 lines graphics
+    ; 160 lines graphics
     ; ANTIC mode e (160px 2bpp, 1 scan line per line)
     .byte $4e
     .addr framebuffer