First I right click on the image and save it to my desktop. Then, on the Google Images page (I'm on a desktop PC using Firefox), on the right side of the search box there is an icon of a camera. If you hover over it, it says "Search by image." Click on the camera.
Then click on "upload an image," then click "browse," and navigate to the picture on your computer. Select the file and click "open."
The results will be the places where the image appears. Lots of them are not helpful in trying to find a cake maker or a recipe (e.g., Pinterest, wedding sites that just gather pictures of pretty cakes), but if you are diligent, you usually find the one you want.
It bothers me that these cake pictures are captured and reposted all over the web without giving credit to the cake maker.