ArcGISDynamicMapServiceLayer PNG32

[RudolfKopriva1]

In the API 10.1 the Dynamic Layer export operation seems to be called with PNG format. I need to call it with PNG32. Can I set the default format in the code or can I configure the service so it will always return PNG32?
I see there are changes in the API 10.2, the format is changed when the service has 'supportsDynamicLayers' set to true. How is it related with the format?

[ErwinSoekianto]

Why do you need PNG32 specifically?

If you want to set transparency, you can use SetOpacity()

[RudolfKopriva1]

The server seems to set the opacity automatically when the export is called with PNG32, see the difference in attache image.
How can I get the correct opacity value for the layer?
Thank you.