Automatic Website Update Nightmare

Firstly, can I stop automatic updates happening and what can happen in the future if I don’t update WordPress?

yes. Here is the WP codex guide on how to:
https://codex.wordpress.org/Configuring_Automatic_Background_Updates

Secondly, can I just select and replace my background images for each of my pages…is it as simple as that?

sorry, i am not sure if i understand .

Have you tried:
– Flushing any caching plugins you might be running, as well as server and/or browser caches. Also, any op cache or content network cache, as well as Cloudflare, clear those caches.
– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– If you can’t get into your admin dashboard, try resetting the plugins folder by FTP. Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– switching to the unedited default Theme (Twenty Sixteen, etc.) for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins). If you don’t have access to your admin area, use FTP , or your web-host’s cPanel or whatever file management application your host provides (no Dashboard access required). Navigate to /wp-content/themes/ and switch to the default theme by renaming your current theme’s folder by adding “-old” to the end of the folder name. Alternately, you can remove other themes except the default theme. That will force your site to use it.
– If all the above steps do not resolve the issue, then try MANUALLY updating. Download a fresh copy of the WordPress .zip file to your computer, unzip it, and use that to copy up all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory. You may need to delete the old wp-admin and wp-includes folders and files on your server before uploading the new ones. Read the Manual Update directions first.
– Backup: If you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong.

where exactly do I place WP codex info to turn off auto updates?

The codex guide I linked in my previous reply has all the info you need.