Guidelines: Art Bulletin Style Guide
Unless you are directed to use another style manual, the College Art Association's Art Bulletin guide is the standard for art historical writing.
MLA Style Guide
Another guide is that issued by the Modern Language Association including a section on citing web sources.
Binghampton University Library's Citing Sources
A summary of issues and links to a great many style sites
See Undergraduate > Resources > Writing Papers > Plagiarism for issues of academic integrity.