Try to download image from image URL, but I get HTML instead

I have the URL https://art42.tumblr.com/random to a webpage that displays some images. I want to download the main image from that page.

If I right-click the image in Firefox (or any other browser) and I choose "Open image in new tab", I get the image (jpg). However, if I try to download the image using the code below, I get an HTML file.

I guess the problem has to do with the Referer. I tried to set the "Referer" parameter to the page's URL, and also to the image's URL, but I still get HTML instead of JPG.

Why can I download the image in Firefox, but can't in my code?

function DownloadFile(CONST Url, Referer: String; OUT Data: TBytes; PostData: String= ''; SSL: Boolean = FALSE): Boolean;   { TESTED OK }
VAR
  Buffer     : array[0..High(Word)*4] of Byte; { Buffer of 260KB }
  TempBytes  : TBytes;
  sMethod    : string;
  BytesRead  : Cardinal;
  pSession   : HINTERNET;
  pConnection: HINTERNET;
  pRequest   : HINTERNET;
  Resource   : string;
  Root       : string;
  port       : Integer;
  flags      : DWord;
  Header     : string;
begin
  Result := FALSE;
  SetLength(Data, 0);
  pSession := InternetOpen(nil {USER_AGENT}, INTERNET_OPEN_TYPE_PRECONFIG, nil, nil, 0);

  if Assigned(pSession) then
  TRY
    { Autodetect port }
    port:= UrlExtractPort(URL);
    if port = 0 then
      if SSL
      then Port := INTERNET_DEFAULT_HTTPS_PORT
      else Port := INTERNET_DEFAULT_HTTP_PORT;

    { Root }
    Root:= UrlExtractDomainRelaxed(Url);
    pConnection := InternetConnect(pSession, PWideChar(Root), port, nil, nil, INTERNET_SERVICE_HTTP, 0, 0); { The second parameter of InternetConnect should contain only the name of the server, not the entire URL of the server-side script. }

    if Assigned(pConnection) then
    TRY
      if (PostData = '')
      then sMethod := 'GET'
      else sMethod := 'POST';

      if SSL
      then flags := INTERNET_FLAG_SECURE  OR INTERNET_FLAG_KEEP_CONNECTION
      else flags := INTERNET_SERVICE_HTTP OR INTERNET_FLAG_RELOAD; // INTERNET_FLAG_RELOAD= Forces a download of the requested file, object, or directory listing from the origin server, not from the cache.;

      Resource := UrlExtractResourceParams(Url);  
      pRequest := HTTPOpenRequest(pConnection, PWideChar(sMethod), PWideChar(Resource), nil, nil, nil, flags, 0);  { The third parameter of HttpOpenRequest is the file name (URL) of the script }

      if Assigned(pRequest) then
        TRY
           Header:= '';
           if Referer > ''
           then Header:= Header+ 'Referer: ' + Referer + sLineBreak;
           Header:= Header+ 'User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0'+SLineBreak;
         //Header:= Header+ 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36 Edg/91.0.864.59'+SLineBreak;  //  Microsoft Edge UA string
           Header:= Header+ 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'+SLineBreak;
           Header:= Header+ 'Accept-Language: en-us,en;q=0.5' + SLineBreak;
           Header:= Header+ 'Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7'+SLineBreak;
           Header:= Header+ 'Keep-Alive: 70'+ SLineBreak; { In windows, default is 60 sec }
           Header:= Header+ 'Connection: keep-alive'+ SlineBreak+SLineBreak;

           HttpAddRequestHeaders(pRequest, PWideChar(Header), Length(Header), HTTP_ADDREQ_FLAG_ADD);

           Result:= HTTPSendRequest(pRequest, NIL, 0, Pointer(PostData), Length(PostData));     { The actual POST data is the forth parameter }
           if Result then
             REPEAT
              ZeroMemory(@Buffer, SizeOf(Buffer));   

              { Download bytes }
              InternetReadFile(pRequest, @Buffer, SizeOf(Buffer), BytesRead);

              { We stop? }
              if BytesRead= 0 then break;

              { Convert static array to dynamic array }
              SetLength(TempBytes, BytesRead);
              Move(Buffer[0], TempBytes[0], BytesRead);

              { Merge arrays }
              Data:= Data+ TempBytes;
             UNTIL BytesRead= 0;
        FINALLY
          InternetCloseHandle(pRequest);
        END
      else
        RaiseLastOSError;

    finally
      InternetCloseHandle(pConnection);
    end;
  finally
    InternetCloseHandle(pSession);
  end;
end;

In your "Accept" header you are not specifying any graphic formats.

A properly configured web server should send you a representation of the resource in one of the formats you accept.

Try looking at the request headers sent from your browser (use the Developer Tools - see your browser's help for how to access them). It will be accepting graphic formats as well as text.