When painting from a photograph which is something I do a lot, one needs to plan.  With time and practice these edits become automatic.  One needs to think about how the vase of flowers is to be laid out on the canvas. One needs to consider the negative space between the subject and the edges of the plane and, most importantly, the positioning of your focal point. This is where the rule of thirds needs to be considered in order to place the important elements of a painting. Study the negative space around the subject and then plan to make it varied and balanced. It is also important to examine the positioning of light and dark shapes, and to avoid placing areas of high detail and contrast in the middle of the canvas.  Photo editing software is wonderful because it allows you to make all sorts of adjustments like the enhancement of colour, light, contrast, and saturation. Alternatively, you can desaturate the colour, lower the contrast and soften the areas that you don’t want to draw attention to. 

And then when you begin painting your subject you can simplify and remove other aspects of the photograph to create a painting you are happy with. 

Ultimately your painting is an interpretation of the photograph you are working from. How you interpret that photograph is what makes your painting. I use a photograph as an idea and inspiration rather than something to copy directly.