1. Upload your image file(s) to /media/import
2. Update your field mapping and add image, small_image, thumbnail
3. Update your CSV file (just do this in Notepad to avoid any issues): add your images at the end of the line, separated by a comma, preceded by a backslash and enclosed in double-quotes. I.e. you will need to type: ,"/myimage.gifâ,"/mysmallimage.gifâ,"/mythumbnail.gif"