Paintings in Museums and Public Art Galleries:
Liechtenstein Museum, Vienna Archduke Karl with his Staff at the Battle of Aspern, 1820
Johann Peter Krafft in the Louvre Museum Database, Paris (only available in French)
Akademie der Bildenden Künst, Vienna (in German) 
Joconde Database of French Museum Collections (in French) 
Professional Tools:
Artprice
Pictures from Image Archives:
Johann Peter Krafft in the Art Renewal Center
Fine Arts in Hungary 
Johann Peter Krafft in the Web Gallery of Art 
Wikimedia Commons Image Database 
Farbdia Archiv (Color Slide Archive) (in German)  Thousands of photographs of fresco cycles and wall decorations in churches and other buildings in and around Germany. Click "Galerie" for a normal overview; click "digilib" to zoom in.
Articles:
Union List of Artist Names (Getty Museum) Reference sheet with basic information about the artist and pointers to other references.
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