Paintings in Museums and Public Art Galleries:
Fine Arts Museums of San Francisco Distant View of Rome, 1891
Musée d'Orsay Collection Database, Paris NEW! 17 works online, out of 99 in the database, at last count. Page forward to view works with images.
Joconde Database of French Museum Collections (in French) 
Palazzo Braschi, Rome, Italy Works in the database (can be a little slow)
State Museums of Florence Digital Archive, Italy
The Essence of Line: French Drawings from Ingres to Degas
Professional Tools:
Artprice
Pictures from Image Archives:
Ernest Hebert in the Art Renewal Center
Ernest Hebert at CGFA 
Ernest Hebert at ArtMagick
Bildindex der Kunst und Architektur (in German) 
Articles:
Union List of Artist Names (Getty Museum) Reference sheet with basic information about the artist and pointers to other references.
La Tribune de l'Art (in French)  Several paintings illustrated in this article on the Musée Hébert near Grenoble in France
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