function GetGIFSize(const FileName: string): Types.TSize;
type
  // GIF header record
  TGIFHeader = packed record
    Sig: array[0..5] of AnsiChar;     // signature bytes
    ScreenWidth, ScreenHeight: Word;  // logical screen width and height
    Flags: Byte;                      // various flags
    Background: Byte;                 // background colour index
    Aspect: Byte;                     // pixel aspect ratio
  end;
  // GIF image block header record
  TGIFImageBlock = packed record
    Left, Top: Word;      // image top left
    Width, Height: Word;  // image dimensions
    Flags: Byte;          // flags and local colour table size
  end;
const
  cSignature: PAnsiChar = 'GIF';  // gif image signature
  cImageSep = $2C;                // image separator byte
var
  FS: Classes.TFileStream;      // stream onto gif file
  Header: TGIFHeader;           // gif header record
  ImageBlock: TGIFImageBlock;   // gif image block record
  BytesRead: Integer;           // bytes read in a block read
  Offset: Integer;              // file offset to seek to
  B: Byte;                      // a byte read from gif file
  DimensionsFound: Boolean;     // flag true if gif dimensions have been read
begin
  Result.cx := 0;
  Result.cy := 0;
  if (FileName = '') or not SysUtils.FileExists(FileName) then
    Exit;
  FS := Classes.TFileStream.Create(
    FileName, SysUtils.fmOpenRead or SysUtils.fmShareDenyNone
  );
  try
    // Check signature
    BytesRead := FS.Read(Header, SizeOf(Header));
    if (BytesRead <> SizeOf(TGIFHeader)) or
      (SysUtils.StrLComp(cSignature, Header.Sig, 3) <> 0) then
      // Invalid file format
      Exit;
    // Skip colour map, if there is one
    if (Header.Flags and $80) > 0 then
    begin
      Offset := 3 * (1 shl ((Header.Flags and 7) + 1));
      if Offset >= FS.Size then
        Exit;
      FS.Seek(Offset, Classes.soFromBeginning);
    end;
    DimensionsFound := False;
    FillChar(ImageBlock, SizeOf(TGIFImageBlock), #0);
    // Step through blocks
    FS.Read(B, SizeOf(B));
    while (FS.Position < FS.Size) and (not DimensionsFound) do
    begin
      if B = cImageSep then
      begin
        // We have an image block: read dimensions from it
        BytesRead := FS.Read(ImageBlock, SizeOf(ImageBlock));
        if BytesRead <> SizeOf(TGIFImageBlock) then
          // Invalid image block encountered
          Exit;
        Result.cx := ImageBlock.Width;
        Result.cy := ImageBlock.Height;
        DimensionsFound := True;
      end;
      FS.Read(B, SizeOf(B));
    end;
  finally
    FS.Free;
  end;
end;
//delphi/2246