Get users to use the download button instead of right-click save

As part of our system, a web application, we have an image viewer. Previously we always displayed the full resolution image but scaled down with CSS to fit the page. For performance reasons we now display a properly scaled version of the image with the option to download the full resolution image with a button under the image.

As part of the users' workflow, they sometimes need to download this full resolution image to submit to another system. The problem is that many users just right-click the image and press "save image as". This used to work fine but now it will instead provide them with a low resolution image. The users don't notice this and this causes problems further down the line. How can we force users away from this behaviour? Sure, we could tell them not to do it but that doesn't feel very scalable. My least bad idea so far is to hijack the right-click in the browser but this is A) not super nice UI and B) not sure to cover all cases.

In short: How can we get users to use the download button instead of saving the low-res thumbnail?