How to re-size our OLD/existing pictures?

[CZ Eddie]

Thanks to XF 2.3, all new images uploaded are auto-resized on the client side.
But does anyone know how I can resize all of our old pictures?
We've got tons of old pics up to 14MB in size that I'd love to downsize down to a MB or less.
How to re-size our OLD/existing pictures?

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