I think the “string” Bill mentioned is the green elastic cord and yellow hook. What you do with a photo depends on the use you are going to put it to. If it is for an editorial or news piece, there are usually rules prohibiting most editing. You can brighten, change contrast a bit and crop. Most other changes are not allowed. If it will be used as a fine art piece, you have complete latitude to change whatever you wish. If it is a fine art piece, cloning out the cord and hook would be worth considering. In the same light, I might consider removing the links of that chain. But, I have no idea why it is there, or what it is. To those more knowledgeable about horses, there may be a good reason to leave it in the photo.
To answer your question, if you want to draw attention to something, add light. If you want to hide something remove light, or add light to other areas to put what you want to hide into shadow. In the case of the chain, I would just remove it with a clone tool, or leave it there if it makes sense.