Sometimes CSS is very playful. It knows that you are new and it teases you.
In this post, I am saying most of the same things that others have already said.
What editor are you using to write your web page?
Do you know the difference between uppercase letters and lower case letters? (some alphabets have no case.)
Can you show the relationship between the folders (directories) in your web site? In particular, (1) what is the name of the folder where the HTML file is located (that is probably the root directory), (2) from the root directory, where is the folder (what is the path) where the CSS file is located, and (3) from the root directory, where is the folder where the image file is located?
Your paths might look like one of these:
Then, into your next post, copy and paste the code from the CSS file that points to the banner image. Be sure it is an exact copy! check spelling, case and spaces.
Then be sure that the spelling of the image, banner.png, is the same as the the spelling in the CSS, and be sure the path to the image matches the path from the CSS file to the image. Any dots or backslashes in front of the path in the HTML and CSS matter, so be sure to copy them exactly.
In the top of the HTML file, there should be a path to the CSS written in the head of the page. Please copy and paste that link exactly.