ActiveX buttons lost pictures after converting to a 2007 file form

Hi,

I frequently use a workbook with different ActiveX buttons on the sheet with
macro's attached to them. I used little icons as pictures on the buttons. In
Excel 2003 and 2007 there are no problems if I use the XLS format. But when I
save the workbook as a 2007 file format (such as XLSB, XLSM) some buttons
lost their pictures. Everytime I open the workbook again and save it, another
button has lost it's picture. Untill there are only grey buttons without
pictures left.

If you have any idea what is causing the problem, your help would be very
much appreciated.

Thanks,
Michael