When converting C function prototypes to Pascal equivalent declarations, it's important to substitute the C function and argument types with the correct Pascal types. I often use a Type Translation Table for this, like the following in table 1 (optional parts on the C-side are noted between square brackets):

It's also nice to have access to the translation of the special WINDOWS.H types, which is already done by Borland, and can be found in WINTYPES.PAS (it's a rewarding experience to compare these two files with each other - you'll learn a lot about C and Pascal conversion issues). 
Now that we've handled the standard built-in (and Windows) types, let's look at the C type definition of WING_DITHER_TYPE that is present in the WING.H file.

 typedef enum WING_DITHER_TYPE
 {
     WING_DISPERSED_4x4,
     WING_DISPERSED_8x8,
     WING_CLUSTERED_4x4
 } WING_DITHER_TYPE;

This is a so -called enumerated type, and can be translated into an ObjectPascal enumerated type very easily:

 type
   WING_DITHER_TYPE =
    (WING_DISPERSED_4x4,
     WING_DISPERSED_8x8,
     WING_CLUSTERED_4x4);

While most C DLL header files contain only constant definitions (of the form #define XYZ value) and function prototypes, type definitions are also found, and are therefore important to be able to convert.