When you create your own HTML webpage and use images that were created by another person, you are required to give credit to them for taking the image so that you could use it.
You are done with the hard part about images. Citing is very easy: first, you go back to the image you found on google, and click license details (only present for creative commons licenses). This will lead you to creative commons. Right above the title, you will find three circles, and next to that, you will find some text similar CC BY-SA 4.0. Copy-paste that text into an <h6> tag right under your image.