If you are looking for original artwork created by artists and not some off the shelf prints, then you should check out Vango, formerly ARTtwo50. The iPhone and iPad app allows you to browse and purchase artwork from independent artists who post their work for sale on the art network.
Vango is more than just a database of paintings, its offers personalizations that are tailor-made to your tastes. The app has a recommendation engine that learns from the art that you favorite. The more paintings you like, the better the app will become at selecting pieces to match your taste. The recommendation engine also takes into account the colors of the room you are looking to decorate, eliminating outlying art work that may be beautiful, but just doesn't suit your current needs.
Vango also offers a visualization feature that'll take a piece of art you are viewing and allow you to use your camera to virtually picture how it would look in your home or office. It's a very useful feature for getting a rough approximation of how the piece will match your decor.
If you enjoy supporting local artists, Vango also has a localization feature that grabs your location information and shows you artwork from nearby artists. You can use this feature to find artists in your hometown as well as find local talent when you traveling away from home.
For artists, Vango allows you to list your artwork starting at $100 and $250 price points, with higher amounts available as you begin to sell your work and build a satisfied customer base. A "following" feature allows your customers to follow you and receive notifications when you upload new pieces of art, sell your work, or move up to a new price level.
Vango is available for free from the iOS App Store. The latest version of the app adds support for the iPhone, making it a universal app that supports both the iPhone and iPad. It requires iOS 7.