Adding PAPD in again right off the bat - it's too universally handy in design research to not mention. A good first stop in any design endeavor!
If you're looking for images, either as examples to draw upon for an architectural period or as inspiration for your own abstract costume design, Artstor is absolutely critical for design oriented theatre research.
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Formerly Grove Art Online, this is the definitive encyclopedia for the arts and design. This portal to Oxford art reference products includes cross-referenced articles and links to art images in galleries and museums. Articles are scholarly and include bibliographic references. This database contains Grove Art Online (the entire text of The Dictionary of Art), The Oxford Companion to Western Art, the Encyclopedia of Aesthetics, and The Concise Oxford Dictionary of Art Terms, and is continually updated.