The Max Plank Institute for the History of Science has posted a page of "Recommendations Concerning the Free Use of Visual Media for Scholarly Purposes", which includes links to a great deal of related information, including their recommendations for best practices.

The page concludes with this:

This document calls upon curators and scholars to enter into a new relationship to promote mutual trust and common interests. The aim of our compact is to address the pressing challenges raised by our digital present and future. We request that curators refrain from arbitrarily restricting the public domain. We further ask our colleagues in libraries, museums, and other repositories to accommodate the needs of scholars for freely accessible, high-resolution digital images. This request concerns not only print publications, but also new forms of electronic publishing. We exhort scholars in the humanities to respect the special custodial responsibility of museums, libraries, and other image repositories. In particular, we insist that careful attention to attribution must become part of each scholar’s contribution to a relationship based on trust and mutual benefit.