How to insert a banner depends very much on which WordPress theme you’re using.
If you want specific help, then you need to give more specific information:
Ideally link to the site and an explanation about where you would like to put the banner (and what size).
Or at least tell us which theme you’re using.
This kind of theme related questions are usully better to put in the support forum for the theme you’re using. Then you may get help from other people, who’re using the same theme, or even from the theme developer.
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