How can I customise the display of an event's associated images?
This is really a CSS question, but you can slice it several ways. All solutions will require a bit of understanding or expertise.


CSS only: You could add CSS to a custom site theme which sets the event background based on the event ID. (For example, the event info page contains a region with class .crm-event-id-3 you could target.) This may be less flexible if you don't want to make image changes in your CSS, but it is fine if you can set-and-forget, or don't mind changing text files. This is what I'd typically do - I find CSS an effective tool.
Smarty, CSS via Custom TPL: Depending on your needs, you might find you want to use Event information to set CSS inline (eg, if you wanted to be able to use CiviCRM to change images via the web UI). To achieve this you might provide custom templates for CiviCRM. Documentation is at Customize Built-in, Profile, Contribution and Event Registration Screens and Page Templates. Note that the first page there recommends the next method! I would avoid this because working with Smarty makes me sad, and for backgrounds you'll end up working in CSS anyway.
Smarty, CSS via Extension: The docs above recommend using an extension to customise the Smarty templates or inject custom CSS. This is the "new" means to achieve the above, but may have a higher barrier to entry. It is better to maintain. I'd do this if, for example, client request was "make it possible for staff to change background per event". I'd still avoid Smarty and just inject CSS to the page.


Three options there! How you deliver this really depends on what your design requirements are, but I hope the pointers are some help.