If the shirt pull has the effect of holding up an opponent, it is punishable by a direct free kick.
In Law 12.1:
A direct free kick is awarded if a player commits any of the following offences:
If the shirt pulling also breaks up or interferes with a promising attack, the player is cautioned for unsporting behaviour.
There are different circumstances when a player must be cautioned for unsporting behaviour including if a player:
- commits a foul or handles the ball to interfere with or stop a promising attack
If the shirt pulling denies an obvious goalscoring opportunity, the player is sent-off, no matter where the offence occurs.
A player, substitute or substituted player who commits any of the following offences is sent off:
- denying an obvious goal-scoring opportunity to an opponent moving towards the opponents’ goal by an offence punishable by a free kick (unless as outlined below)
Where a player commits an offence against an opponent within their own penalty area which denies an opponent an obvious goal-scoring opportunity and the referee awards a penalty kick, the offending player is cautioned unless:
- The offence is holding, pulling or pushing or
- The offending player does not attempt to play the ball or there is no possibility for the player making the challenge to play the ball or
- The offence is one which is punishable by a red card wherever it occurs on the field of play (e.g. serious foul play, violent conduct etc.)
In all the above circumstances the player is sent off.
In the photograph you've provided, any competent referee would consider that level of shirt pulling to be holding. It should have been punished with a penalty kick, since it occurred in the penalty area.
Like any other foul, even if shirt pulling does hold an opponent, if it's not seen by the referee or another member of the officiating team, it will go unpenalised.
Also, like any other foul, if stopping play for the free kick would harm the fouled team more than allowing play to continue, the referee will wait a few seconds to see if advantage can be played. If within that few seconds, the attacking team end up with a better opportunity than the free kick would have afforded, play continues. This does not preclude the referee from returning to warn or caution the offender when the ball is next out of play as shown here.