Many botany websites welcome public contributions to their educational collections of flower and plant pictures, including Dave's Garden, the USDA Plants Database, Phyto Images and PlantSystematics.org. These databases consist mostly of high-resolution images that are suitable for printing and are free to browse or download.
The databases are organized by plant family and provide classification information for botany enthusiasts. Each site has its own rules concerning usage and licensing, but most follow Creative Commons or similar copyright agreements that permit individual and non-commercial use, making them a good bet for the classroom.
For high-resolution images, the USDA Plants Database offers large photos of over 50,000 plants. Each photo has a button to download the file directly, which saves time when obtaining photos to print. For sheer size, Dave's Garden wins first prize, with a database of over 200,000 plants created by its 58,000 members. Free membership is open to the public, and users also swap gardening tips on the community forums.
Phyto Images and PlantSystematics.org have mid-sized collections of 16,000 and 10,000 plants respectively. The galleries host high-resolution images, and the collection at PlantSystematics.org includes illustrations as well. Both of these educational botany sites welcome image contributions from the public, though they have specific parameters concerning file type, quality and size.