You may find the stuff in the middle of your photo has done some wierd space-time stretchy-business and your great-niece's cute little button nose has morphed into an unholy alliance with Grocho Marx's. Even worse, it might be subtle. More than once I've started a painting using a photo as a reference and halfway through I'm thinking, “What's wrong with this picture?” It's distortion, or else the truth about your great-niece's nose has been hidden from you through the eyes of love. I'll vote for distortion.
So before you start blocking in that painting, really scrutinize your photo. Look at the area closest to the camera and see if you can percieve a sort of visual swelling. Compare areas in the foreground to areas in the background and see if there's a wierd size difference. And if you're taking photos, stand well back from your subject. If you need detail, take two photos – one from far back to get the whole scene, and other zoomed in to get details. And watch that nose.