A photograph taken totally out of context, no matter how convincing it appears, can often lead to the completely wrong conclusion. It is of course obvious that there are visible injuries, as in this case, but the photo most certainly cannot verify how the person came by their injuries.
The Police "react" to situations as opposed to "instigating" them, and often have to resort to strongarm tactics in order to achieve their goal. The vast majority of Joe Public have no concept of how difficult it is to restrain a suspect and eventually arrest him if he does not comply.
Not everyone "comes quietly", and it is not uncommon for a suspect to incur some sort of injury if he puts up some kind of resistance or acts violently towards the Police. Once an Officer informs a suspect that he is under arrest, he cannot just turn away if the person begins lashing out. There is no easy or magical way to restrain someone acting like that without to some extent having to use brute force and physical force, because it is often impractical and inappropriate for example to use CS gas or indeed a taser if he arrest is taking place, say, in a house.
If anyone doubts my comments, either join the Police and find out for yourself, or watch closely most nights of the week in a town centre near you! You'll soon get the idea.