The Source Image Display options determine how the raster image is displayed after One-Touch Vectorisation. You can select either:
Retain Original Image. With this option the raster image remains unchanged after One-Touch Vectorisation. This option requires enough extra memory to store a copy of the raster image, because the processing is perfromed on the copy, so as to leave the original image unchanged. If your PC is very low on memory, selecting this option may make the processing much slower, because it may cause Windows to use the swap space memory on the hard disk.
Use Processed Image. With this option the raster image is not copied, but all processing is performed on the original image. Therefore when the One-Touch Vectorisation has completed you will see the thinned raster image, and the original image is discarded.