Assume stdin format is png

src/main.rs

@@ -1,7 +1,7 @@
 use clap::Parser;
 use eyre::{Context, ContextCompat, bail, ensure, eyre};
 use image::{
-    DynamicImage, GrayAlphaImage, ImageReader,
+    DynamicImage, GrayAlphaImage, ImageFormat, ImageReader,
     imageops::{flip_vertical_in_place, overlay},
 };
 use nix::ioctl_write_ptr;
@@ -15,6 +15,8 @@ use std::{
 #[derive(Parser)]
 struct Opt {
     /// Path to the image-blob. "-" means stdin.
+    ///
+    /// When reading from stdin, format must be PNG.
     pub path: PathBuf,
 
     /// Stamp an image onto the final image. "<x>:<y>:<path>"
@@ -70,7 +72,9 @@ fn load_wallpaper(path: &Path) -> eyre::Result<GrayAlphaImage> {
         eprintln!("Reading image from stdin");
         let mut buf = vec![];
         stdin().read_to_end(&mut buf)?;
-        ImageReader::new(Cursor::new(buf)).decode()?
+        let mut reader = ImageReader::new(Cursor::new(buf));
+        reader.set_format(ImageFormat::Png);
+        reader.decode()?
     } else {
         eprintln!("Opening image at {path:?}");
         ImageReader::open(path)?.decode()?