Would painting a photo out of National Geographic Magazine/Website count as a form of plagiarism?

actually, yes. you're violating the copyright of the photographer. you can do it for practice. but if you are going to exhibit the work, or sell it, you have to alter the image significantly to use it without permission of the photographer. That includes just using a small detail, or, changing the shapes, colors, composition, or somehow significantly altering the meaning of the picture.