There is a wide range in quality of the fabric photos between different websites. My favorite feature is the zoom over, where you can scroll over the photo and it is greatly enlarged so you can see the details of the print. A couple of my favorite fabric vendors have recently 'improved' their websites, and this feature was eliminated. So frustrating.
I often do a google search to see other pictures of a fabric if I can't make out enough detail on the website I'm buying from, or if I really need it to be a specific color. I think when buying online, you always have to be prepared that the fabric might look different than anticipated.