I continue answering in this thread...
The functionality for loading the external bitmap is necessary for the target system (e.g. STM32Fx) - here you need to implement the function EwLoadExternBitmap() as C code.
In order to provide this functionality within Embedded Wizard Studio (this means, to provide the same functionality within the Prototyper) a so called Intrinsic Module is necessary. In principle this is a Windows DLL loaded into Embedded Wizard Studio to add a certain functionality.
Please have a look into the provided example: There you will find the C module externbitmaploader.c which is used for the target - and you will find the file ExternBitmapLoader.ewi. This file (*.ewi = Embedded Wizard Intrinsic) is the Windows DLL that provides the extern bitmap loader functionality for the Prototyper.
You will also find a Visual Studio Project in order to build the Intrinsic module.
There is an alternative to keep things more simple:
If you do not need to work with dynamically loaded images within the Prototyper, then just use a regular bitmap resource in case of Prototyper and use the extern bitmap resource only in case of target.
This can be controlled by using the build-in macro $prototyper.
Does this answer your question? If not, let us know some more details what you finally want to achieve.