There is an ever-growing list of museums that are making high-resolution images of works no longer under copyright, available to be used without restriction. Check individual museum sites and "terms of use" to see how their images can be used. Make sure image quality standards offered by individual sites comply with quality standards requested by publishers (file size, clarity, sharpness, etc.). Below is a selective list of such providers.
Lisle
Kindlon Hall
5700 College Rd.
Lisle, IL 60532
(630) 829-6050
Mesa
Gillett Hall
225 E. Main St.
Mesa, AZ 85201
(480) 878-7514