A new technique for ionospheric imaging is demonstrated during a severe geomagnetic storm of 15th July 2000. The three-dimensional time-dependent imaging algorithm is applied to multi-directional ground-based GPS data and yields spatial maps of electron concentration. This technique is demonstrated by showing a series of images of the mid-latitude ionosphere over the USA during the storm of July 2000. A strong uplift in the height of the F-layer is observed after 20 UT on 15th July, followed by a severe latitudinal electron concentration gradient. Independent verification of the images is provided by the ionosondes and incoherent scatter radar data. The GPS images reveal the large-scale dynamics of the ionosphere during the disturbed conditions and show the potential of geophysical imaging for storm-time ionospheric studies.