Museum and gallery lighting suppliers can furnish you with a projector type imaging fitting which will project an adjustable square or rectangular patch of light so that only the picture is lit and the surrounding wall is left relatively dark. These are used frequently in museums and commercial art galleries and make the picture look great but aren't cheap. The two most important things to consider when choosing any lighting for art are a high CRI (colour rendition index) from the light source and low UV emission. A high CRI lamp (or lamp and lens combination) will render colour and tone correctly while low UV will assist with archival permanence of your images.