Dreamstime's website is possibly the best for searching an displaying of relevant results. The website is fast and easily usable by all. However, there are some tips that may help you to better refine your search results.
The first tip has to be to visualize what kind of image you want. The chances are that you'll be able to find exactly what you want (or better). Think about whether it's a photograph or an illustration, then use the check boxes to the right of the search box to ensure you only get back what you want.
Paging through the results can take quite some time, so why not set the "items per page" to 60, instead of the default 20. This will brink back more results each time you page through the results.
Finally, if you like an image, click on the name of the photographer/artist and you may get a whole host of other similar images that better suit your project.
Hope these tips have helped. Good luck in your search for the perfect image!