Cloud:
http://openclipart.lynms.edu.hk/clipart/foom/foom_cloud.svg

Gas cloud:
http://www.clker.com/clipart-cloud-25.html

Cow:
http://www.clipartsfree.net/svg/12763-chewing-cow-vector.html

Factory: 
http://www.downloadclipart.net/download/5857/factory-svg

Power station:
http://www.clipartlogo.com/image/conventional-power-station_291969.html