Try this

default.nix

@@ -2,6 +2,6 @@ with import <nixpkgs> {};
 stdenv.mkDerivation {
     name = "media";
     buildInputs = [ imagemagick
-                    (python3.withPackages (ps: with ps; [ pyyaml ]))
+                    python3Packages.pyyaml
                   ];
 }

scripts/resize-thumbnails.sh

@@ -15,6 +15,6 @@ for x in *; do
     large=$x
     small=../thumbnails-small/$x
     if ! [ -f $small ] || [ $large -nt $small ]; then
-	convert $x -resize x"$thumbnail_height" $small
+	magick $x -resize x"$thumbnail_height" $small
     fi
 done