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procedure WB_ClickImage(WB: TWebbrowser; ImageSRC: string);
var
  iDoc: IHtmlDocument2;
  i: Integer;
  iDisp: IDispatch;
  iColl: IHTMLElementCollection;
  InputImage: HTMLInputImage;
begin
  if Assigned(WB.Document) then
  begin
    WB.Document.QueryInterface(IHTMLDocument2, iDoc);
    if Assigned(iDoc) then
    begin
      iDisp := iDoc.all.tags('INPUT');
      if Assigned(iDisp) then
      begin
        iDisp.QueryInterface(IHTMLElementCollection, iColl);
        if Assigned(iColl) then
        begin
          ImageSRC := AnsiUpperCase(ImageSRC);
          for i := 1 to iColl.Get_length do
          begin
            iDisp := iColl.item(Pred(i), 0);
            iDisp.QueryInterface(HTMLInputImage, InputImage);
            if Assigned(InputImage) then
            begin
              if Pos(ImageSRC, AnsiUpperCase(InputImage.src)) <> 0 then
              begin
                InputImage.Click;
              end;
            end;
          end;
        end;
      end;
    end;
  end;
end;					

1:

WB_ClickImage(Webbrowser1, 'go.gif');