Do image quality decrease with time
No, photo quality itself doesn't decrease with time but if you copy and save same image multiple time then compression algorithm will be running every time to reduce file size. In such case you will see your image quality decreasing as you save the same image multiple times.
If you have stored your good quality photo of JPG or JPEG few years ago then you will fill that the photo quality is not good enough today.
What do you think, Do photo quality decrease with time ?
No, its absolutely not. Today due to advancement of technology the camera quality is increasing day by day. Few years ago when you have saved your photo, you have never seen a camera better than that of your photo. But today you are using advanced camera. In such case, if you compare your old photos with of today you will feel that the image quality has been decreased. But, image quality has not decreased but camara quality of today's generation has increased.