Change custom Office ribbon size on window resize

Question

In our Office add-in we need the ribbon buttons to behave the same way as regular Office ribbon buttons do when you resize the form: as the form becomes smaller, some images also become smaller, then disappear altogether. I have no idea how to do the same for our custom ribbon group, the buttons always stay the same. Please note, this is Office UI ribbon, not WPF ribbon, which is much more customizable

Thanks!