We illuminate art in different ways, and this can largely depend on size and the type of art in question. If the piece has glass, then you need to be careful when illuminating to avoid the light reflection so choose something that sits a bit further away, often a ceiling mounted spot light.
If we are using a light that is more traditional in style then we would usually design this to be fitted to the back of the picture rather than the wall so there is flexibility for art be changed at a later date. Don’t be overly worried about ensuring you have chosen all your art and picture lights in the early stages of your project when you are planning the layout but do highlight key spots throughout your home for art and future proof by asking your electrician to fit ‘clock points’ that a picture light can be fitted to at a later date.