This was a very EARTHy riddle for Earth Day. Each picture clues a word made of the letters of EARTH. The duplicate letters in each word—HEATER, EARHART, HATTER, HEARTH—spell EARTH again. (In a different order, they also spell HEART, which is something else you have only one of, and so was also accepted.)
Congrats to solvers Leo S., Dr. Yetman, and the Gregg household!