Art is very personal and should be chosen on the basis of what really speaks to you, and for lasting pleasure genuine oil paintings/watercolors/acrylics are much more satisfying than prints. I've found a lot of nice things on e-bay, and those offered by the artist are often less expensive than dealers. You might consider larger pieces on each side of the fireplace and put together a grouping of smaller pieces of what appeals to you on the longer white wall. In a grouping there should be some kind of common thread that ties them together whether it's type of art, similar framing, size etc., although it's fun to mix things up a little. A good way to style them is to lay all your pieces out on the floor and play around with them until you find a satisfying composition. For the art on the blue wall bigger is always better.