Adding a timestamp to an uploaded image

In Rails 3 we have the asset pipeline, that automatically takes care of
images, caching, etc.

But what about images uploaded by the user? Those images are uploaded
into
the “uploads” folder, and while you can move them around, they are
outside
the asset pipeline (please correct me if I’m wrong).

So, the user uploads an image and when he replaces it with another one,
the
browser still displays the previous one because it’s using the cache, so
a
page refresh (cmd+r) is needed to display the new one.

How to avoid that? Isn’t the image_tag supposed to add the timestamp
automatically?