I found out what the problem is: when looking at the helloworld_with_VSTGUI project now provided with SDK 3.6.9 I saw that there was the same file with different names: background.png, background_2x.png, background_3x.png which led me to believe that somehow the name of the bitmap contains the resolution…
Renaming my image from Control_AudioSwitch_50x100.png to Control_AudioSwitch.png fixed the problem…
I am very unclear how and when the _2x, _3x extension is being used and would definitely love to get some info on that…
Yan