Images, sprites, objects, etc would be loaded as normal in any piece of code you add to your project.
Generally an extension to a project comes in 2 files, .h and a .c or .cpp. The .c or .cpp gets compiled into an object file that gets linked to your main project. It holds your code, compiled just the same as the main file but can make compiling time quicker.
The .h file usually holds references to the functions in the .cpp or .c file, just like you would forward declare a function or even data.
The GDK (DBP) end exe file is what is generated just like in DBP so ID's are just the same across all included files. Meaning you can call a function in another .cpp file with the ID of an image and it will work hunky dory.
Unless you make a typo in your code...
Mental arithmetic? Me? (That's for computers) I can't subtract a fart from a plate of beans!
Warning! May contain Nuts!