Hi Philipp,
When you pass an ICO file to an Image control (which we use in the Ribbon's controls), then it will always pull the first frame or icon. So if your ICO has a 32x32 icon and a 16x16 icon, then the latter icon will never be used. In this case, the 32x32 icon will be scaled down to 16x16, or scaled up depending on where/how it's used.
In 2009.2 build 0513, we added the IconFrameSelector markup extension to our Shared Library. You can use it in place of your ICO uri, like so "{shared:IconFrameSelector <existing Uri>, DesiredHeight=16, DesiredWidth=16}". It will then try to find and use an icon of the specfied size, and fall back to the default behavior if one is not found.
If this doesn't appear to be your issue, then please put together a small sample project that reproduces the issue and email it over to our support address.