Click the “img” button above, you’ll get a pop up that says paste the url of the image. That url has to end in .jpg or .png or .gif or some other file format that this site recognizes as an image. I think google photos can add other junk to the url sometimes so it might not be the best option for hosting. People also run into permission/sharing issues with google drive at times.
In the past I created a google site and posted my images to a page there, which allowed me to get an actual url of the image. If it’s just sitting in your google photos within google drive it’s going to give you a goofy nonsense url that will get you to the photo but will not be recognized as an image by this site or others.