The Los Angeles County Museum of Art (LACMA) has just launched its new Image Library, which lets you download free, high-resolution images of works from its permanent collection. Many other museums already do the same thing, but LACMA’s Image Library is different in one significant respect: it explicitly permits you to use its images without restriction. That means you don’t need to seek the museum’s separate permission in order to use these images in publications.
Note that this only applies to works that LACMA has established as being in the public domain, so most 20th-century works are excluded. The VRC’s Copyright page explains why this is.
You can read LACMA’s Terms and Conditions of Use here. Hopefully, this openness is part of a trend towards greater access to images.