These are a few websites that I like for their collections or skill in art and painting content, also friends' sites too.
My Etsy online shop - 'Gilded Serif Prints'
Many of my prints at various sizes are for sale here and growing daily! https://etsy.me/39fOkTR
Home to the heart of British Art Collectiing http://www.nationalgallery.org.uk/
National Portrait Gallery
If you like portraits, then this is the place to be http://www.npg.org.uk/
It doesn't get any better than this for the British Art, the "crème de la crème" www.racollection.org.uk
C'est fantastique! http://www.louvre.fr/llv/commun/home.jsp?bmLocale=en
The edgy side of UK and International Art http://www.tate.org.uk/modern/
Love it or loathe it, it's art to talk and debate about http://www.saatchi-gallery.co.uk/
Current contemporary artists www.portlandgallery.com
Well worth a peek http://www.guggenheim.org/
Superb master of contemporary landscape art www.peterwilemanartist.co.uk
Charles Harrington - American Artist
Charles Harrington a master of painting with light http://charlesharrington.com/
A great collection and the home of Hals' "Laughing Cavalier" http://www.wallacecollection.org/