Hi Naomi,
New Years was great around here. Hope you have enjoyed 2016 so far.
Great questions. There are two ways you could add the banner to your site. You could use a plugin like DW Promobar (https://wordpress.org/plugins/dw-promobar/). These add a banner to the top of your site and may be a good option for you. There are lots of them out there. I don’t remember specifically if you can add an image with this one. If you can’t you could make your banner a background image if necessary for the banner’s bar.
The other option would be to edit the header.php file. If you’d like to have the banner outside of the “wrap” that the site sits in, you can add in the HTML for it above this line:
<div id="wrap" class="clearfix">
If you’d rather have the banner be the full width of the site’s content area and not extend beyond that, then place it just below that line.
The content area of the site is 960 pixels wide so that would be the width you’ll want to use. The height is variable just depending on what you need. The image should resize automatically based on screen size so you should be good to go with mobile too.
Let me know if you have any questions.
Take care,
Bill