Hi! I doubt there's one site that encompasses
all the information (for 'all' artists) that you're
seeking, however, Artcyclopedia.com is a very
good site and can act as a springboard to others.
Just remember... the internet is filled with
misinformation... always get at least 2-3 sources,
i.e. a museum, a book... and there are many good
books online! Look to the Smithsonian archives
for American art, the Joconde for French, the UK
National Inventory for Continental Paintings..
but there is no formula. Try to find academic
sources whenever possible.
http://www.bridgemanart.com/image.aspx?key=80262
http://www.flickr.com/photos/32357038@N08/6063159894/
http://www.bridgemanart.com/image/Zais-Giuseppe-1709-84/Landscape-with-a-River-and-Dancing-Peasants-oil-on-canvas/1ca5d317061d43ca89933c269b4fe37f?key=Landscape%20with%20a%20River%20and%20Dancing%20Peasants%20by%20Giuseppe%20Zais%20&filter=CPOIHV&thumb=x150&num=15&page=1
http://www.google.com/search?q=acap&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&biw=920&bih=495&source=lnms&ei=mEOTTqCUHq2rsALTsPmwAQ&sa=X&oi=mode_link&ct=mode&cd=1&ved=0CC4Q_AUoAA#pq=galleria+d%27arte+moderna+florence++fontanesi&hl=en&sugexp=tsh&cp=14&gs_id=5&xhr=t&q=Giuseppe+Zais+lithograph&pf=p&sclient=psy-ab&client=firefox-a&rls=org.mozilla:en-US%3Aofficial&source=hp&pbx=1&oq=Giuseppe+Zais+&aq=0v&aqi=g-v4&aql=f&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=b12f5e501efbd0e5&biw=920&bih=495
http://www.artcyclopedia.com/artists/zais_giuseppe.html
http://www.google.com/search?q=acap&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&biw=920&bih=495&source=lnms&ei=mEOTTqCUHq2rsALTsPmwAQ&sa=X&oi=mode_link&ct=mode&cd=1&ved=0CC4Q_AUoAA#sclient=psy-ab&hl=en&client=firefox-a&rls=org.mozilla:en-US%3Aofficial&tbm=bks&source=hp&q=Antonio+Fontanesi+&pbx=1&oq=Antonio+Fontanesi+&aq=f&aqi=&aql=&gs_sm=s&gs_upl=4734l4734l2l6256l1l1l0l0l0l0l148l148l0.1l1l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=48f8c01b6b57e07f&biw=920&bih=495
http://www.google.com/search?q=acap&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&biw=920&bih=495&source=lnms&ei=mEOTTqCUHq2rsALTsPmwAQ&sa=X&oi=mode_link&ct=mode&cd=1&ved=0CC4Q_AUoAA#sclient=psy-ab&hl=en&client=firefox-a&rls=org.mozilla:en-US%3Aofficial&tbm=bks&source=hp&q=Giuseppe+Zais+&pbx=1&oq=Giuseppe+Zais+&aq=f&aqi=&aql=&gs_sm=s&gs_upl=65869l65869l4l66977l1l1l0l0l0l0l351l351l3-1l1l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=48f8c01b6b57e07f&biw=920&bih=495
Art resources online:
http://www.google.com/search?q=Leroy+Neiman+Napoleon&hl=en&client=firefox-a&rls=org.mozilla:en-US:official&biw=920&bih=495&prmd=imvnso&source=lnms&ei=XV6TTuLvCoGJsgKWtpyuAQ&sa=X&oi=mode_link&ct=mode&cd=1&ved=0CDQQ_AUoAA#hl=en&client=firefox-a&rls=org.mozilla:en-US:official&sa=X&ei=2GKTTr-PF4iLsAK5nY2xAQ&ved=0CBcQvwUoAQ&q=art+resources+online&spell=1&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=b12f5e501efbd0e5&biw=920&bih=495
Hope this helps!