Artchive - Spanish Art
Good resource for high quality images by El Greco, Velazquez, Goya, and Picasso, but you won't find many other artists represented. The archive contains both public domain and copyrighted works, but the copyrighted works are free to use under the Copyright Provision Act of the Copyright Law. You are free to use up to 5 or 6 images for slideshows, websites, etc.

Look Into:
World Architecture Survey - Renaissance Spain
World Architecture Survey - Early 20th c. Spain