It depends on the source. Some artwork catalogues are available that are VERY easy to work with - they provide you with the art and minimal specs, and you provide them with a licensing fee for whatever level of useage. Some are very reasonable.
Private art will be harder to deal with - it'd be a matter with negotiating with whatever collection owns the piece. The group I'm in just went through this with the Met in NY - a long but overall reasonable process got us the artwork.
Is this what you mean?
M