When the FreeImage is loaded, there has to be allocated data internally to address the various plugins. The initialisation functions are responsible for that task. Note that in the precompiled DLL version the initialisation functions are automatically called. You normally don't need to initialise FreeImage (but it doesn't hurt when you do)

void FreeImage_Initialise();

Initialises the library.

void FreeImage_DeInitialise();

DeInitialises the library.