I was working on a power point for my art class on watercolour. After checking out many images on Google and saving several examples including images from Winslow Homer and Helen Frankenthaler....I remember a website that many of my students constantly refer to....www.deviantart.com...I found a lot of images there that span many styles, interests and age appropriate materials. I am always amazed when I visit this website as it celebrates the artistic endeavours of so many styles and interests. I am sure that most of your keen art students have already been there and know about it and I am sure that other students will benefit from looking at art that has been created by millions of artist many of whom art their age.
Also, this website is a huge resourse for artists both traditional and digital. You can download brushes and textures, become a part of groups that share common iterests, etc.
Check it out!