Painting from Photographs instead of from life.
During my college years I had a professor who adamantly insisted that [realist] art should always come from real life. Figures from live models, Still-life's from crafted actual still life settings, landscapes from actual, uh, scapes of land... He was training us like the old masters used to learn, the way he had been trained in art. After all, the concept of photography has only been around for 190 years, and digital photography has only existed for 24.* Before people could snap a photo on their phone and pull it up later to sketch, filter, alter and post, before the Polaroid was a revolution, then a trend and then a hipster phenomenon, all realistic art was drawn from real life.
In a school setting - especially a 'school of art' setting - these things are fairly easy to come by and should be utilized if at all possible. The school will often provide live models, both nude and clothed, for life drawing, figure painting and some sculpture classes. There are secure spaces to set up an elaborate still-life - with perfectly reproducible lighting - and leave it for weeks while layers of oil paint dry. Landscapes.... well, they stay pretty much the same whether or not you're paying tuition - baring bad weather or natural disaster, at least.
Outside of a school setting however... the options for a "from life" artist become more limited. Normal, non-art people will find you pretty creepy if you ask them to take off all their clothes and hold a pose in your living room/studio for several hours at a time. Even if you aren't set on a nude model, your fully clothed friends will still fidget, get sore and annoyed and eventually be done with sitting for you. (Seriously, art modeling is so much harder than it looks... have you ever held the exact same pose for 3 hours? Yoga enthusiasts and Buddhist monks please apply!) Try setting up a still-life in a normal household and it will be moved, molested, altered or destroyed in mere moments by the chaos of everyday living...
Sometimes Photos make art easier. Soooooo much easier.
One of my most triumphant moment in college was during a class-wide critique, where we laid out our current works for the professor to judge, condemn and grade. I nervously presented a painting that had started from a live model and ended using photographs, as my model/friend had gotten super fidgety, even with the promise of free pizza as motivation... only to excitedly realize that my professor COULDN'T TELL. He couldn't tell that I had "cheated" and done 80% of the painting sans live model. From that point, I was all about [sneakily] painting from photographs. I would set up my still-life or pose my model, take photos from different angles for reference, then dismantle or dismiss. The reference photos didn't complain or move about, the lighting never faltered, and If I wanted to paint at 3 a.m., I could, with no bother to anyone else. Lovely.
NOTE: I should mention that I enjoyed the challenge of painting cloth and clothing, so all of my photographed models were dressed... I don't know if a nude model would be as accepting of photos... that is an agreement to be reached between artist and model.
Painting from photographs is both glorious and convenient, but can also be problematic.
The Problem with painting from photographs is that you only see what you have captured in the photo, a collection of lines and colors on a flat surface, restricted to one perspective.
If you conveniently happen to be trained in painting from real life - and more specifically, trained by someone who believed in teaching the human form in gesture, mass and shadow before lines - the issue is more easily conquered. You look at the image and can envision the actual space, the person sitting, the pile of nick-knacks that you arranged to recreate. You've had training, so you can translate what you see in the 2D image into what it is in reality, an object that occupies a 3D space, features that are wrapped around a 3D human form, influenced in minute ways by unseen muscle and bone that are nowhere to be found in that simple photograph.
Recently, a few of my artistically inclined friends, talented but untrained in life-drawing techniques, have asked me for advice in creating realistic portraits in ink and paint. They're working entirely from photos and are running into problems that I remember encountering at the very beginning of my time in life drawing classes. Both are already incredibly talented artists in other mediums, but in the attempt at portraiture they run into little snags... faces look a bit flat or 'off'... the eye's aren't quite right... the picture was taken at an odd angle and the perspective is messing the lines up... it looks like a person, but not recognizably the person they were going for...
The majority of these issues in portraiture can be avoided by realizing that what you are attempting to paint or draw is not a flat oval with almond shapes for eyes and line defined features. Those features are soft... gentle flesh supported by a ridged frame of cartilage and bone. They are built onto a 3D shape, they dip in and fold over and emerge out into the space around them. The eyes tuck in because the are literally orbs in deep sockets, with flaps of skin stretched over ridges of bone to keep them safe. Yes, everyone has an upper lip, but it is usually MUCH less pronounced than the lower, sometimes just a shadow of the angle. Not every part that you know is there can be shown with a hard line. It's a creepy concept to dwell on, but we're all just bones, held together by tendon strips, wrapped in meat, protected by skin and covered in hair. Creepy, but entirely true.
There are some art colleges that require students to dissect human cadavers in order to fully understand what is going on under the skin. This is the exact method the old masters used to learn the secrets of accurately reproducing the human form on a 2D surface, unseemly - and illegal - as it was at the time. Dealing with an actual body teaches a student definitively that people are not a series of hard lines, but a carefully orchestrated gathering of mass.
I am by no means encouraging grave-robbing, or telling anyone without a full-fledged art education that realistic portraiture is a hopeless goal. There are those in the world who have an insane natural talent and completely understand how a 3D body occupies space. The figure, portraits and photo-realistic art come easily for a luck few. For the rest of us, it is critical to remember that the structure is as important as the surface of a person or object, especially if you are attempting to draw a 3D object from a 2D image.
Don't give up on drawing people! Don't give up if you have trouble painting an object to look like that object! Draw from life, draw from photos, draw from sketches and memories. It will get easier. The human body/face will look like you mean it to. Humans are hard to capture, emotions even harder. Only with practice and perseverance will they start to make sense. Just remember what the photograph IS: a fully 3D structure, and then apply what the photograph is OF: color, light and shadow on the surface of that structure.
Bring the photo back to Life.
Resurrect the image.