Force image extension on right click save

I'm having some issues with images on my site which I'm downloading from an image delivery service.

They're displaying perfectly fine, but the issue comes if a user tries to right click and save the image. The images seem to be leaving off the image extension. Is there any way to force the image to download as, say a jpg? I've looked at the download attribute, but I think that's only valid on links, so it's not valid in this case.