Browser cache images sent from Dropbox

I am writing an application that uses Dropbox as the source of image files that are downloaded via javascript by setting img.src = "[Dropbox download link]". This works great, but I often collect a lot of files at a time, and when reloading a set of images (for example, when reloading a page), the browser resends the request for each of them (which returns 304 unchanged). I would like the browser cache to display the image on boot, so I can just avoid repeated requests in general, but I cannot figure out how to enable caching of these images in the browser. Can I set cache headers in this situation? Dropbox response headers have "cache control" set to "no-cache".