How to automate keeping track of images being uploaded and resize them (24/7).

Dropbox is no longer offering the option for creating new long-lived access tokens. Dropbox is now issuing short-lived access tokens, which do expire after a short period of time, and optional refresh tokens, which don't expire, instead of long-lived access tokens. You can find more information on this migration here.

Apps can still get long-term access by requesting "offline" access though, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. For long-term unattended use cases like yours, you will need to request and use a refresh token. After the initial authorization though, once you have the refresh token, you can have your app use it automatically, so that the app can perform Dropbox operations whenever needed.

You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example. Specifically, "Step 5: Retrieve a new short-lived access token" of section 2 "Using offline access" shows how to programmatically use a refresh token to get a new short-lived access token whenever.