Sport art is another lucrative niche. There's a growing trend toward portraying sports figures from golf, tennis, football, basketball and racing in prints which include both the artist's and athlete's signatures. Work in a series. It is often easier to market a series of small prints exploring a single theme or idea than to market single images. A series of similar prints works well in long hospital corridors, office meeting rooms or restaurants.
Hotels often purchase two similar prints for each of their rooms.
If you hope to get published by a commercial art publisher or poster company, your work will have a greater chance of acceptance if you use popular colors and themes. Attend trade shows. Many artists say it's the best way to research the market and make contacts. Increasingly it has become an important venue for self-published artists to market their work. Research art publishers thouroughly before you sign with them.