diff --git a/src/greaseweazle/image/dcp.py b/src/greaseweazle/image/dcp.py
index 00f87095..6c77a190 100644
--- a/src/greaseweazle/image/dcp.py
+++ b/src/greaseweazle/image/dcp.py
@@ -1,4 +1,4 @@
-# greaseweazle/image/dim.py
+# greaseweazle/image/dcp.py
 #
 # Written & released by Keir Fraser <keir.xen@gmail.com>
 #
@@ -8,24 +8,27 @@
 import struct
 
 from greaseweazle import error
-from greaseweazle.image.img import IMG
+from greaseweazle.image.img import IMG_AutoFormat
 from greaseweazle.codec import codec
 from .image import Image
 
-class DCP(IMG):
+class DCP(IMG_AutoFormat):
 
     read_only = True
 
+    @staticmethod
+    def format_from_file(name: str) -> codec.DiskDef:
+        fmt = codec.get_diskdef('pc98.2hd')
+        assert fmt is not None # mypy
+        return fmt
+
     def from_bytes(self, dat: bytes) -> None:
 
         header = dat[:162]
         pos = 162
-        format_str = 'pc98.2hd'
-        fmt = codec.get_diskdef(format_str)
-        assert fmt is not None # mypy
+        fmt = self.fmt
 
-        for t in self.track_list():
-            cyl, head = t.cyl, t.head
+        for cyl, head in self.track_list():
             if self.sides_swapped:
                 head ^= 1
             track = fmt.mk_track(cyl, head)
@@ -38,7 +41,6 @@ def from_bytes(self, dat: bytes) -> None:
                 self.to_track[cyl,head] = track
             elif header[cyl * 2 + head] != 0:
                 raise error.Fatal("DCP: Corrupt header.")
-        self.format_str = format_str
 
 # Local variables:
 # python-indent: 4
diff --git a/src/greaseweazle/image/dim.py b/src/greaseweazle/image/dim.py
index ad637246..5b11b764 100644
--- a/src/greaseweazle/image/dim.py
+++ b/src/greaseweazle/image/dim.py
@@ -8,17 +8,20 @@
 import struct
 
 from greaseweazle import error
-from greaseweazle.image.img import IMG
+from greaseweazle.image.img import IMG_AutoFormat
 from greaseweazle.codec import codec
 from .image import Image
 
-class DIM(IMG):
-    default_format = None
+class DIM(IMG_AutoFormat):
+
     read_only = True
 
-    def from_bytes(self, dat: bytes) -> None:
+    @staticmethod
+    def format_from_file(name: str) -> codec.DiskDef:
+
+        with open(name, "rb") as f:
+            header = f.read(256)
 
-        header = dat[:256]
         error.check(header[0xAB:0xB8] == b"DIFC HEADER  ",
                     "DIM: Not a DIM file.")
         media_byte, = struct.unpack('B255x', header)
@@ -28,10 +31,16 @@ def from_bytes(self, dat: bytes) -> None:
             format_str = 'pc98.2hs'
         else:
             raise error.Fatal("DIM: Unsupported format.")
+
         fmt = codec.get_diskdef(format_str)
         assert fmt is not None # mypy
+        return fmt
+
+    def from_bytes(self, dat: bytes) -> None:
 
         pos = 256
+        fmt = self.fmt
+
         for t in fmt.tracks:
             cyl, head = t.cyl, t.head
             if self.sides_swapped:
@@ -40,7 +49,6 @@ def from_bytes(self, dat: bytes) -> None:
             if track is not None:
                 pos += track.set_img_track(dat[pos:])
                 self.to_track[cyl,head] = track
-        self.format_str = format_str
 
 # Local variables:
 # python-indent: 4
diff --git a/src/greaseweazle/image/img.py b/src/greaseweazle/image/img.py
index 622de239..071cfc62 100644
--- a/src/greaseweazle/image/img.py
+++ b/src/greaseweazle/image/img.py
@@ -10,7 +10,7 @@
 from greaseweazle import error
 from greaseweazle.codec import codec
 from greaseweazle.flux import HasFlux
-from .image import Image
+from .image import Image, OptDict
 
 class IMG(Image):
 
@@ -91,6 +91,20 @@ def get_image(self) -> bytes:
         return tdat
 
 
+class IMG_AutoFormat(IMG):
+
+    @staticmethod
+    def format_from_file(name: str) -> codec.DiskDef:
+        raise NotImplementedError
+
+    @classmethod
+    def from_file(cls, name: str, fmt: Optional[codec.DiskDef],
+                  opts: OptDict) -> Image:
+        error.check(fmt is None,
+                    f'{cls.__name__}: Format cannot be overridden')
+        fmt = cls.format_from_file(name)
+        return super().from_file(name, fmt, opts)
+
 # Local variables:
 # python-indent: 4
 # End:
diff --git a/src/greaseweazle/tools/write.py b/src/greaseweazle/tools/write.py
index ad6f8a27..d917b677 100644
--- a/src/greaseweazle/tools/write.py
+++ b/src/greaseweazle/tools/write.py
@@ -235,10 +235,6 @@ def main(argv) -> None:
         print("Writing " + str(args.tracks))
         if args.precomp is not None:
             print(args.precomp)
-        if hasattr(image, 'format_str'):
-            print("Image format " + image.format_str)
-            error.check(args.format is None,
-                        'Cannot override image format with --format')
         if args.format:
             print("Format " + args.format)
         try: