While the Baltimore Grotto, an organization founded in 1952 and dedicated to “the exploration, study and conservation of caves,” does use the phrase “take nothing but pictures, leave nothing but footprints, kill nothing but time” as its motto, the origins of the phrase, which varies in usage, are unknown. That said, the idea behind the phrase, is deeply rooted in ecological conservation and simply refers to the need to minimize one’s impact on nature, including refraining from removing plants, rocks, or other natural things from protected areas.
Humanity’s interaction with the Earth’s natural environment has been very destructive at the individual, corporate, and national levels. Industrial pollution is well-documented, as is the destruction to the natural environment carried out by many governments around the world seeking to exploit natural resources. Less appreciated is the destructive impact of individuals and small groups on the environment. The phrase “take only pictures, leave only footprints, kill nothing but time” is intended to address this "micro" level of humanity by impressing upon tourists, hunters, and others the need to respect nature and to minimize the adverse impact upon nature caused by tourists, hunters, hikers, and others. “Leave nothing but footprints” means clean up your waste from the lunch or snacks your brought along on your adventure. “Kill nothing but time” means refrain from killing wild animals and plants whose natural habitat you are currently invading.