http://www.artbusiness.com/ provides complete art consulting and advisory services, art appraisals, art price data, articles for artists and collectors market research and general information.
www.paintmyphoto.ning.com devoted to sharing reference photos to inspire derivative artworks without infringeing copyright.
http://www.photos4artists.co.uk/ a similar but smaller resource than the Paint My Photo site. Unfortunately no search facility at present. The 'Purely Background' gallery is useful.
http://www.wetcanvas.com/ long established online artistic community catering to all levels. Very comprehensive site - exploring fully would seriously cut into your painting time.
http://improvemypaintings.com/index.html - another veteran internet site hosting tips, advice and master classes