That's actually very helpful to know, what about if you can't find the original source of a picture?
There are image search tools you can use to help you find the URLs. You can find a list of them as well as other useful information about proper sourcing in help:image_source.
Be careful, though. Just because an image looks identical on a webpage doesn't mean that webpage is the original source. You should also check the image data itself. You can use https://duplicatebooru.zipfiled.info to compare the URL of the image with the one you posted on Danbooru to make sure they are the same file, though I personally also download both images and compare them byte-by-byte with a hex editor just to be safe.