I had never thought of doing that. I just tried, and clicking the + symbol in WhatsApp gives me option to send photos/video, documents, contact, and location.

If I choose documents, I do not see where I can go to select photos/videos in the iPhone camera roll. I presume it may not be accessible that way.

Also, I presume WhatsApp does not want to allow people to send photo/video at full size in their network to reduce bandwidth costs.