This squat is not anywhere close to the correct depth*. Go into any gym, anywhere, and almost every guy will be squatting just like the picture -- way too high and probably using the same weight.
Most books and trainers call for squatting to parallel. This is the standard in powerlifting, where a legal squat requires the hip crease to drop below the knee. This is fine, if you actually do it.
In reality, parallel squatters cheat**. I know, because I've done it myself and I've seen literally hundreds of other people do it. In this style of squat, the last few inches above parallel are the most difficult part of the movement. Add heavy weights and the temptation to cheat becomes nigh irresistible. If you don't force yourself to go well below parallel, you will squat high.
My solution: squat to full depth on every rep. This means descending as far as you can on each rep, feeling the hamstrings make contact with the calves.
Yeah, I know, full squatting is overkill, you can't use as much weight, some stuff about your knees, whatever. Based on what I've seen, though, I think the average trainee would get more benefit from squatting if we treated the full depth version as the "regular" squat and saved the parallel version as a special case.
By law, all blog posts about squatting must include this picture
The exact form of any exercise will vary from person to person, so there is no one true "correct" way to squat. That being said, here are some tips I gave Chelsea when she was learning the movement:
Take a comfortable stance. There is no reason for regular people to go uncomfortably wide or extremely narrow***.
The bar sits in a natural groove on the traps. Some authors teach a very low bar position borrowed from powerlifting. This can give you better leverage, but stresses your shoulders.
Toes straight ahead or turned out a little, whatever feels best.
Keep your chest up and back flat throughout the movement. A little forward lean may happen, but don't allow yourself to suddenly drop forward, especially on the ascent, where it may make you miss the lift.
If you have trouble going deep, try raising your heels with a small plate. If this helps, you need to improve your ankle mobility.
In general, strive to make your movements natural and fluid. Deep squatting requires full lower-body mobility, so this may reveal some areas where you're stiff or inflexible.
Finally, contrary to what some authors say, you don't have to squat. The squat is good exercise, but it isn't mandatory. You can get excellent results without doing heavy squats.
* Partial squats are a legitimate exercise, particularly if you do them with very heavy weights. The great Paul Anderson did a lot of high squats as assistance for his full squats. Letting your normal squats turn into high squats is the problem.
** They don't necessarily want to cheat, it just sort of happens.
** Powerlifters like a wide stance to decrease the range of motion. Bodybuilders may use a narrow stance to emphasize the quads.