If the image is as common as you say and it is located on several different websites for different companies or different things and the diagram does not have a registration mark (r with a circle around it), TM in one of the corners, or some other mark to indicate ownership, then it is not trademarked. In the US, searches can be done by the general public on the federal website. There is bound to be a government entity in your country that issues trademarks and copyrights. I would look there under the FAQs. If you are still uncomfortable after reading the information on these websites, then you may want to check in with a lawyer that is licensed to practice in you country. (My education in law does not reach as far as Australian law)Best of luck!