If you want to use an image that you already found but you can't remember where it came from, you can try a reverse Google search.
Look to the right for the word Images; click on it
Look to the right for the camera icon; click on it
Copy the image URL or upload the image from your computer/phone.
The Google results should show you where an image originated. Remember attribution decay though. Just because the image is showing up EVERYWHERE on the internet, still doesn't mean you can use it. You certainly need to - at the very least - link back to where you got the image from.