The competition is designed to highlight the U.S. National Parks, and the entries must be realistic (to some degree) representations of scenes from one of the 390 areas supervised by the National Parks Service (the inclusion of National Monuments and Historic sites expends the possibilities considerably).
The current entry form hasn’t been posted yet, so I don’t know the entry fee, but you can download a Regions Map from this page that lists the eligible National Parks, Monuments and Historic sites.
There are prizes for each of three national regions, and a grand prize, a $10,000 Purchase Award. There is also a Mini Competition with a $5,000 Grand Prize for paintings of 180 square inches or less. There are more details here.
Winning paintings will be be featured in a touring exhibition and online, providing additional exposure.
There is a PaintAmerica blog, on which you can find more information, as well as see paintings from last years winners from both competitions.
(Image above: Robert Gunn, Thomas Marcotti, Robert Highsmith)