Should this goal have been ruled offside? For what it's worth, it was called offside by the referee.
This goal is absolutely regular (according to offside rule)
When the cross is kicked there are no players in offside. Then the goalkeeper deflects the ball over the bar (or the ball arrive directly over the bar), and in this moment (if the video screenshot is at the right moment) there is only a player in offside (near the left bar of the goal "numer 1"), but this player is a not-active position.
So the player that score the goal (number 3) is in a regular position for offside rule because the last defender is over (number 2).
In my opinion watching the video is more easy to understand than looking the screenshot
No, the goal should have been allowed. The referee was incorrect in calling offside here.
Contrary to what has been suggested in other answers, the only time offside position should be evaluated for the purposes of determining when an offence has occurred is when the ball is last touched by a teammate.
p. 36 of the 2015/16 FIFA Laws of the Game (emphasis mine):
It is not an offence in itself to be in an offside position.
A player in an offside position is only penalised if, at the moment the ball touches or is played by one of his team, he is, in the opinion of the referee, involved in active play by:
- interfering with play or
- interfering with an opponent or
- gaining an advantage by being in that position
If this was not the case, attackers would not ever be able to run past the second last defender and challenge the goalkeeper for the ball like this and this. Note in both of these incidents, the scoring player was in an offside position when the ball was last touched by the goalkeeper.
Therefore, the only time that offside position should be judged is when the cross was played into the area, as shown in the following screenshot.
At this point, you will notice that there are two defending players closer to the goal line (the goalkeeper, and the white defender standing directly in front of him) than every other black player.
p. 36 of the 2015/16 FIFA Laws of the Game
A player is in an offside position if:
- he is nearer to his opponents’ goal line than both the ball and the second-last opponent
As none of the black team are offside when the ball was last touched by a teammate, they are all free to challenge opponents and play the ball until it is next touched by a teammate (when each player's offside status will be re-evaluated).
This should be a goal as pointed out by Ale due to the player 2 (red in Ale's picture).
The referee did not probably see the player 2. The referee's decision not a goal probably relies on the part 3 of offside rule if player(s) offside at the point of the crossing shot. Otherwise I would count this as a regular goal but the lineman's flag means that there must have been a player(s) in offside position before the crossing shot.
The point 3 of the Wikipedia link above is bolded below
Interfering with play, not satisfied.
Interfering with an opponent, can be argued to be satisfied.
Gaining an advantage by being in an offside position
"playing a ball that rebounds or is deflected to him off the goalpost, crossbar or an opponent having been in an offside position; or that rebounds, is deflected or is played to him from a deliberate save by an opponent having been in an offside position" (FIFA, Interpretation of the Laws of the Game and Guidelines for Referees)
where the referee probably concluded that the third is satisfied.