Britannica Image Quest may be your answer! The web address is http://quest.eb.com. Your librarian will have your username and password. If you have Mackin Via, the students should be able to access this database - and it should be automatically authenticated! This database has nearly 3 million rights-cleared images from more than 50 of the best collections in the world. You can search in Spanish as well as English.
Check it out to use with your next project! You can save pictures straight to the camera roll on an iPad.