I am trying to add a header to the board.
I don't know how to add it. If I put it anywhere on the index.php it gives errors.
If I add it above it covers the page.
Anything, that get's displayed in the browser is part of the generated document and is written in HTML. You tried to apply your changes to parts the program code instead and that's the wrong place.
You have to alter the HTML main template
main.tpl, you'll find in the directory
default with the name of another theme, if you applied one). What to do in HTML depends on your use case. For defining the final dimensions and the position of the image you have additionally to work with the rulesets in CSS, written down in
themes/default/style.min.css (and for the purpose of documentation with a better readability in
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