Ok - solved, I was mislead...
I looked at the Accounts(31) example and build my own buttons in the very same way. In Accouunts(31) the Buttons type is shown as "Submit", hence the problem in my project. The generated HTML for ym button was a <input... After changing the Type to "Button" everythings works well, even PNG's are displayed.
Strange thing anyway, the display of "Submit" as type in the Accounts example.
Regards, thomas