Using Dropbox to automatically upload images to a website.

That certainly would be possible, using either the Core API or Chooser.

If you want a more automatic flow, you can use the Core API . First, you'd need to register an app. You can find information on the different permissions here. For this, the app folder permission would likely be sufficient, unless you want the user to be able to specify a different folder.

Then, you'll need to implement the app authorization flow. The OAuth guide should be helpful there. We also highly recommend using one of the official SDKs, if possible, as they'll do most of the work for you.

Once the user is linked to your app, your app can use the /files (GET) API call to access file content from the user's account.

You can also use webooks to get low-latency notifications of changes to linked user accounts, so you would know when to download files. There's a sample app linked on that page that overlaps a bit in functionality here.

Alternatively, you could use the Chooser, but it would require a bit more manual intervention from the user. That is, they would need to go to the website, trigger the Chooser, and select the file(s) to send. (As opposed to the Core API implementation above, where the user would just need to drop files into the app's folder on any linked device, which could even be automated.) Using the Chooser would likely be easier to develop though.

If you did go the Chooser route though, the basic idea would be to use the "direct" link type, and send that link to your server, where you could make a network request to download the file from the link.