You can upload images from the web on Web App like imgur.
For example, take your image (url), paste it on https://imgur.com/upload and then, you can edit your image:
Here you can resize your image:
- Change the width, height will auto resize
- Apply
- Save the changes
You should have the new resized image, take url and apply it to your tribute page CSS.
If your background image is too small, you’ll see that the background image will repeat itself.
You have to use some background CSS properties to fix that (you can read this post)
That’s just an example, you can do so much things with CSS! 
Have a nice day!