When manipulating photographs, it’s important to consider what the purpose the photo is. If for example, the photograph is modified to create artwork, most manipulation techniques are acceptable. Artists are encouraged but not required to include a list of the techniques applied to the original image. When processing photos, adjustment to color brightness and contrast are acceptable. The main point is to modify a photo as little as possible but artists have more freedom as to how to modify their images. Art shows and competitions may have specific guidelines for participants as well so they must be aware of those.
When it comes to journalistic photos on the other hand, there are very strict guidelines on how much a photo can be manipulated before it begins to cross the lines of misrepresentation. As a rule of thumb, adding and removing any person or object to or from a photo is extremely discouraged. Staging the photo is what must be avoided at all cost. Generally, it’s acceptable to adjust the color, brightness and contrast up to a point. Substituting colors is crossing the line. Black and white is ok and sometimes even encouraged. Don’t make it too dark on the edges.