If you use a proper <img> tag to embed an image in a webpage, it will display correctly even if the server sets a Content-Disposition header to force download. The header only makes a difference when you explicitly navigate to the URL of the user-uploaded resource.
But I can imagine many scenarios were the whole purpose of letting the user upload images is so that you can inline display them on the web (such as a forum avatar picture). Requiring the manual downloading of the image wouldn't be an acceptable solution for most people there.