Have you thought about taking an actual photo of your work? I think it would have far greater impact than using a stock image, as well as enhance your credibility and showcase your work.
If you're not too handy with a camera, you might be able to get a local photographer to do it for you at little to no cost if you give them a shout out on your website.
Otherwise, I use www.photopin.com
. It pulls photos from Flickr, so they are "real" pics rather than cheesy stock photos. You just need to be aware of the licensing restrictions and the need to credit the photographer. Also, if the photo is taken by another landscaping company, I would steer clear of it to avoid any misrepresentation.