Thanks for all the answers..!

To MrCode:
The file format PNG is supported by the engine.
(see manual: File formats -> "Supported image formats are PCX, BMP, TGA, DDS, PNG, JPG, and WAD")

I also tried with BMAP....did not work, too.

To Ottawa:
I tried that. Did not work, too.

To hack-panther:
Well I probably have to use a dummy panel for each function I want to call, too.

To Spirit:
I have now tried with a background image and a certain size....it does not work......

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Oldschoolj, you say your panels are working..can you post an example panel?