Hi Horst,
Since you can upload certain extensions, this leads me to think that .png is prohibited in the Images Repository. To check this can you please open your Images Repository and copy and paste what is in the field, "Prohibited File Types". Out of the box it is:
!gif,jpeg,jpg
Because of the "!" in front it does the opposite and makes it so these are the only file types that you can upload. So in the out of the box setting, it will only let you upload files with the extensions: gif, jpeg and jpg. If yours is the same as the out of the box one try adding png to it like so:
!gif,jpeg,jpg,png
and try again.
- Anthony Lee