Richard,
There are IMAQ DLLs that can be called from C++. These would be installed as part of the NI-IMAQ driver software (downloadable from this website). The installation of this software will also install a function reference manual to help you use the IMAQ DLLs.
If you want to do image processing, you would also need the Vision for Measurement Studio package which would install additional DLLs for image processing. This is a separate package that would have to be purchased.
If you have additional questions you may want to post additional details on the task that you are trying to accomplish so that this forum can help you to answer your questions.
Regards,
Jed R.
Applications Engineer
National Instruments