For more generic parks and nature images, try government sites like the ones below.
Creative Commons licenses let you search for materials that creators have already allowed for certain kinds of uses. Most image sites let you search by these kinds of licenses (and Creative Commons has their own search, linked on this page).
In Google Images, you can do a search and then choose Tools underneath the search box and under Usage Rights choose Creative Commons licenses. You'll probably notice a lot of images from free stock photo sites and from Wikimedia, the AV arm of Wikipedia, since all images in Wikipedia have to be freely licensed.
Adding site:gov after your keywords will get you US government-produced public domain images (but make sure to check that the photographer is a US government employee and that the photo hasn't just been licensed for use on the website).
NOTE: Though these aren't all freely licensed, you can also click on the first photos pictured on Google Maps when you search for a location and it will show you lots of other photos geotagged for that location, which you may be able to use via fair use.
Sometimes what you need won't already exist and you'll need to create it yourself. Use the Reeder Media Center to check out equipment, digitize material, use specialized multimedia software like Adobe Creative Suite, reserve a room to make a podcast or music, or learn how to do all the above!
