Get width and height of a GIF image file

function ReadMWord(f: TFileStream): Word;
type
  TMotorolaWord = record
    case Byte of
      0: (Value: Word);
      1: (Byte1, Byte2: Byte);
  end;
var
  MW: TMotorolaWord;
begin
  f.read(MW.Byte2, SizeOf(Byte));
  f.read(MW.Byte1, SizeOf(Byte));
  Result := MW.Value;
end;

procedure GetGIFSize(const sGIFFile: string; var wWidth, wHeight: Word);
type
  TGIFHeader = record
    Sig: array[0..5] of char;
    ScreenWidth, ScreenHeight: Word;
    Flags, Background, Aspect: Byte;
  end;

  TGIFImageBlock = record
    Left, Top, Width, Height: Word;
    Flags: Byte;
  end;
var
  f: file;
  Header: TGifHeader;
  ImageBlock: TGifImageBlock;
  nResult: integer;
  x: integer;
  c: char;
  DimensionsFound: boolean;
begin
  wWidth  := 0;
  wHeight := 0;

  if sGifFile = '' then
    Exit;

  {$I-}
  FileMode := 0;   { read-only }
  AssignFile(f, sGifFile);
  reset(f, 1);
  if IOResult <> 0 then
    { Could not open file }
    Exit;

  { Read header and ensure valid file. }
  BlockRead(f, Header, SizeOf(TGifHeader), nResult);
  if (nResult <> SizeOf(TGifHeader)) or (IOResult <> 0) or
    (StrLComp('GIF', Header.Sig, 3) <> 0) then
  begin
    { Image file invalid }
    Close(f);
    Exit;
  end;

  { Skip color map, if there is one }
  if (Header.Flags and $80) > 0 then
  begin
    x := 3 * (1 shl ((Header.Flags and 7) + 1));
    Seek(f, x);
    if IOResult <> 0 then
    begin
      { Color map thrashed }
      Close(f);
      Exit;
    end;
  end;

  DimensionsFound := False;
  FillChar(ImageBlock, SizeOf(TGIFImageBlock), #0);
  { Step through blocks. }
  BlockRead(f, c, 1, nResult);
  while (not EOF(f)) and (not DimensionsFound) do
  begin
    case c of
      ',': { Found image }
        begin
          BlockRead(f, ImageBlock, SizeOf(TGIFImageBlock), nResult);
          if nResult <> SizeOf(TGIFImageBlock) then
          begin
            { Invalid image block encountered }
            Close(f);
            Exit;
          end;
          wWidth := ImageBlock.Width;
          wHeight := ImageBlock.Height;
          DimensionsFound := True;
        end;
      'ÿ': { Skip }
        begin
          { NOP }
        end;
      { nothing else.  just ignore }
    end;
    BlockRead(f, c, 1, nResult);
  end;
  Close(f);
  {$I+}
end;

var width,height:word;
begin
  GetGIFSize(filename,width, height);
end;