How to assign an image to a variable so that it can be changed in one place and displayed on many pages. – HTML & CSS

I have an HTML & CSS website with many pages and the same photo displayed on top. I would like to find a way to change the photo & its width & height in one place and have it display on those multiple pages without changing each page individually, which is what I am doing now but there are now just too many pages. I am not sure how to do this in either html, ccs or maybe even javascript. is the website if this helps. I tried to google the answer but I think I don’t quite know enough about these languages to know if I even found the answer. – Thanks for any help you can give.