Sites makes it easy to add an image from the web, but it doesn't always make it easy to give attribution. Sites uses two different "Insert Image" widgets. The widget to insert an image on a page (like below), searches the web for images marked for reuse, so you're probably not infringing on copyright under fair use -- but you can't give credit either.
Sites uses the same image search widget for changing the image on the banner for each page (above), but in this case the search results will also display the URL for any image you select.
And it's the URL you need for attribution!
If you ever intend to share a Google Site more broadly, it makes good sense to properly attribute the images you embed. Citing images will not only give credit where credit is due -- invisible labor! -- but also give you a way to follow up should you ever need different or more images in a similar style.
To track down the source of an image embedded on a page via Google Image Search, you can create a dummy section, like this one, and use the more robust "Change Image" widget to locate the same image you have already embedded elsewhere. Use descriptive key words to describe the image you are looking for and use the color-picker to narrow the results further. You can also narrow by "style," limiting the results to photos, clipart, etc.
In the example above, searching for Kermit, cupcake; selecting green for the color; and photo for the style leads to https://pxhere.com/en/photo/494571. There, if you can avoid the distraction of all the advertisements, you'll see that attribution is not required.* At least you now know were to find other images in the series.
In a pinch, you can also take a screenshot of the image and then drag and drop the image into the search field at images.google.com
*Kermit is copyrighted as a character, but images of the frog not used for commercial gain are acceptable.