This mosaic consists of 33 separate images acquired by Voyager 1 as it approached for its close flyby of Io on March 5, 1979. Because it took almost an hour to capture all the images, Voyager 1's point of view shifted throughout the imaging; the individual photos have been "reprojected," stretched and warped based on information about Voyager 1's position while capturing each image to fit a common point of view (in this case, it is an orthographic projection, in which the viewpoint is from infinity).
The mosaic reveals a number of volcanic features: paterae, flow fields, tholi, and plumes, in various shapes and sizes. Paterae in the south polar region tend to be larger than those nearer to the equator, suggestive of differences in lithospheric properties and magma source regions in Io's mantle.
Here are some of the named places visible:
NASA / JPL / Jason Perry
Voyager 1's mosaic of Io's southern hemisphere (labeled)
Original image data dated on or about March 5, 1979