Hi Erwin,
I am afraid, there is no possible to show an image in the floating window in current implementation of our docking manager. It is already known feature and I have reported in the following link:
Add ability to show an image on the caption bar
You could also track the status of the feature request in the above link. If any update will be made on this issue I will inform you. Thanks for using Syncfusion Products.
Regards,
Vijay