I created a project in scratch and shared it, but when my friends search it, they find it but instead of showing a background of the project, it just shows a white background with scratchy the cat in the middle, how do I change the picture on the website

To update the picture, called the thumbnail, whenever your project last saved, whatever exactly the stage looked like will be the new thumbnail. To save it again, just drag a block in and take it out, and you can save again, or go to File > Save Now.