We usually just e mail them the plants WE want them to see. That way if you lead them to a really exciting encyclopedia of plant ideas they don't find unsuitable items when you are so far in to the design process.

We are also creating our own picture library so we can grab a high resolution image when we need it.

Are you using the 'images' tab at the top of the google search results? That will just show pictures.

I use the pics from the MO Botanical Garden website. They are free to use as long as you give them credit for the pics.