hmm actually this may be harder than I thought... The headers are part of the platform SDK. but I'm not sure microsoft still distributes platform SDK's compatible with VC6.
THe newer ones won't work for sure because VC6 isn't c++ standards compliant.
You really should consider upgrading to a newer version of VC.
In any case, even if you do find a platform SDK with GDI+ support that works for VC6. CImage (an MFC class) won't work on VC6. you can use the GDI+ primitives of course.