Two excellent sources for free landform images are Kidzsearch.com and NASA Geomorphology from Space, disc.gsfc.nasa.gov. Both sources include an extensive array of images derived from aerial photography or from space. Copy the images by right-clicking and pasting to your application.
Kidzsearch contains a list of landforms by mechanism of formation, with links to images of approximately 70 percent of the images listed. Each landform image also has a description of what it is and how it was formed. The NASA website contains more than 200 plates, each of which covers a landform type within the geographic region in which it occurs. Each plate is accompanied by commentary, photographs and locator maps. Some also include a map showing bedrock geology. The NASA list of landforms is much more extensive than the Kidzsearch list.