Humm... its not really a new feature, more like a defect, since in this case the new update broke something that works (IMHO):
You're saying that the user should make the choice, but having the browser prompt you to download the image as the default behaviour is a decision the admin made for the user as well.
Unless you are giving the user a checkbox or some option for them to choose their own behaviour, ANY pre-configured behaviour would be a decision made on the part of the admin, and frankly most user don't want to make that kind of choice as long as they don't suffer greatly in the usability department.
And that's where this became a problem because the EXPECTED behaviour when you click on a scaled-downed image in MOST web gallery will take you to an bigger (or original) image WITHIN the browser.
And so this change of behaviour in the new version is BAD, because it BROKE CONVENTION, which in this case breaks usability.
If the user want to download the image instead, he can always right click and choose "Save image as", or right click on the scaled-down image and choose "Save target as..."
I view most download single image function as a duplication of a perfectly good browser function (like those silly JavaScript font-resize buttons...), but if you really want them, you could always add a button that says "download this image" or just "download" instead of modifying the conventional behaviour.