Maybe one of your adlists has added a domain that images are being served from, so now they are all blocked.
If you right-click the missing image in your browser you will have an option to open it in a new tab. You'll be able to see if it's not loading and what the address is. For example on https://support.squarespace.com there are a lot coming from https://theme.zdassets.com. You can test if that's blocked by going to Tools > Search Adlists, typing in theme.zdassets.com and clicking Search exact match.
If you find any domains blocked, whitelist them in Domains > Domain > type in the domain to be added > Add to Whitelist. Then try the the Squarespace support site again and see if images are loading now. You might have to repeat, and the FAQ @jfb mentions covers it.