diff --git a/crates/render/src/paint.rs b/crates/render/src/paint.rs
index 427d39d8a..d021a8f27 100644
--- a/crates/render/src/paint.rs
+++ b/crates/render/src/paint.rs
@@ -65,20 +65,34 @@ pub fn decode_failures() -> u64 {
     DECODE_FAILURES.load(std::sync::atomic::Ordering::Relaxed)
 }
 
-/// `bytes` decoded within [`MAX_DECODE_ALLOC`]; a failure is counted in [`decode_failures`].
+/// `bytes` decoded within [`MAX_DECODE_ALLOC`] ([`decode_within_allowance`]); a failure is counted
+/// in [`decode_failures`].
 fn decode(bytes: &[u8]) -> Option<image::DynamicImage> {
-    let img = image::ImageReader::new(std::io::Cursor::new(bytes)).with_guessed_format().ok().and_then(|mut r| {
-        let mut limits = image::Limits::default();
-        limits.max_alloc = Some(MAX_DECODE_ALLOC);
-        r.limits(limits);
-        r.decode().ok()
-    });
+    let img = decode_within_allowance(bytes).ok();
     if img.is_none() {
         DECODE_FAILURES.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
     }
     img
 }
 
+/// `bytes` decoded within [`MAX_DECODE_ALLOC`], counted as RGBA: the image's size is read from its
+/// header first, and an image of more than that as RGBA gets a limit error before anything is
+/// decoded (#1717).
+pub(crate) fn decode_within_allowance(bytes: &[u8]) -> image::ImageResult<image::DynamicImage> {
+    let reader = || -> image::ImageResult<_> {
+        let mut r = image::ImageReader::new(std::io::Cursor::new(bytes)).with_guessed_format()?;
+        let mut limits = image::Limits::default();
+        limits.max_alloc = Some(MAX_DECODE_ALLOC);
+        r.limits(limits);
+        Ok(r)
+    };
+    let (w, h) = reader()?.into_dimensions()?;
+    if u64::from(w) * u64::from(h) > MAX_DECODE_ALLOC / 4 {
+        return Err(image::ImageError::Limits(image::error::LimitError::from_kind(image::error::LimitErrorKind::InsufficientMemory)));
+    }
+    reader()?.decode()
+}
+
 /// `img` shrunk, keeping its proportions, to fit a pixmap's sides (at most `u16::MAX` pixels).
 fn fit_pixmap(img: image::DynamicImage) -> image::DynamicImage {
     let (w, h) = (img.width(), img.height());
diff --git a/crates/render/src/tests.rs b/crates/render/src/tests.rs
index a5e32ce0d..70b8a4559 100644
--- a/crates/render/src/tests.rs
+++ b/crates/render/src/tests.rs
@@ -799,6 +799,73 @@ fn images_take_their_nearest_pixel_without_smoothing() {
     assert_eq!(greys(false), 0, "nearest: black, then white");
 }
 
+/// A PNG that says it is `w` × `h` 1-bit gray pixels (its header and no rows).
+fn png_header(w: u32, h: u32) -> Vec<u8> {
+    let mut out = b"\x89PNG\r\n\x1a\n".to_vec();
+    let ihdr: Vec<u8> = [&b"IHDR"[..], &w.to_be_bytes(), &h.to_be_bytes(), &[1, 0, 0, 0, 0]].concat();
+    for chunk in [&ihdr[..], b"IDAT\x78\x9c", b"IEND"] {
+        out.extend((chunk.len() as u32 - 4).to_be_bytes());
+        out.extend(chunk);
+        let mut crc = flate2::Crc::new();
+        crc.update(chunk);
+        out.extend(crc.sum().to_be_bytes());
+    }
+    out
+}
+
+/// A little-endian TIFF that says it is `w` × `h` 8-bit gray pixels in one strip (its header and
+/// no pixels).
+fn tiff_header(w: u32, h: u32) -> Vec<u8> {
+    let mut out = b"II*\0".to_vec();
+    out.extend(8u32.to_le_bytes());
+    // (tag, type: 3 SHORT or 4 LONG, value), one value each.
+    let entries: [(u16, u16, u32); 9] =
+        [(256, 4, w), (257, 4, h), (258, 3, 8), (259, 3, 1), (262, 3, 1), (273, 4, 8), (277, 3, 1), (278, 4, h), (279, 4, 0)];
+    out.extend((entries.len() as u16).to_le_bytes());
+    for (tag, kind, value) in entries {
+        out.extend(tag.to_le_bytes());
+        out.extend(kind.to_le_bytes());
+        out.extend(1u32.to_le_bytes());
+        out.extend(value.to_le_bytes());
+    }
+    out.extend(0u32.to_le_bytes());
+    out
+}
+
+/// The decode allowance (2 GB) counts an image's pixels as RGBA, read from its header (#1717): a
+/// 1-bit gray image whose 8-bit gray pixels are within it but whose RGBA is over it is not decoded
+/// and draws nothing, counted for an export to report; one a column narrower goes on to decoding.
+#[test]
+fn an_image_too_large_as_rgba_is_not_decoded() {
+    let over = paint::decode_within_allowance(&png_header(23_171, 23_170));
+    assert!(matches!(over, Err(image::ImageError::Limits(_))), "{over:?}");
+    let failed = decode_failures();
+    assert!(paint::decode_pixmap(&png_header(23_171, 23_170)).is_none());
+    assert!(decode_failures() > failed);
+    // Within the allowance: decoded, and its missing rows can't be read.
+    let within = paint::decode_within_allowance(&png_header(23_170, 23_170));
+    assert!(matches!(within, Err(image::ImageError::Decoding(_))), "{within:?}");
+    // A header whose size as RGBA does not fit in 64 bits gets the same limit error.
+    let huge = paint::decode_within_allowance(&tiff_header(1 << 31, 1 << 31));
+    assert!(matches!(huge, Err(image::ImageError::Limits(_))), "{huge:?}");
+}
+
+/// A gray image within the allowance draws its gray levels.
+#[test]
+fn a_gray_image_draws_its_gray_levels() {
+    use vectorcraft_doc::{ImageBlob, ImageObject, NodeKind};
+    let img = image::GrayImage::from_fn(2, 1, |x, _| image::Luma([if x == 0 { 40 } else { 200 }]));
+    let mut png = vec![];
+    img.write_to(&mut std::io::Cursor::new(&mut png), image::ImageFormat::Png).unwrap();
+    let mut d = Document::new(40.0, 20.0);
+    d.images.insert("gray".into(), ImageBlob::new("image/png", png));
+    let id = d.alloc_id();
+    let im = ImageObject { key: "gray".into(), width: 2, height: 1, xf: Affine::scale(20.0), link: None, placement: Default::default() };
+    d.insert(Some(d.layers[0].id), 0, Node::new(id, NodeKind::Image(Box::new(im)))).unwrap();
+    let r = Renderer::new().render(&d, 40, 20, Affine::IDENTITY, &RenderOptions { smooth_images: false, ..Default::default() });
+    assert_eq!((r.pixel(5, 10), r.pixel(35, 10)), ([40, 40, 40, 255], [200, 200, 200, 255]));
+}
+
 /// Art Optimized exports are supersampled (#983): two red shapes meeting along a diagonal leave no
 /// seam of the white background between them, where shape-by-shape smoothing shows one; the outer
 /// edges stay smooth.