Took me a while to figure out how to simply show an image when locking with xlock:
$ xlock -mode image -bitmap image.xpm -count 1
-mode image: sets the mode to only display random sun images
-bitmap image.xpm: replace the random sun images with image.xpm (*)
-count 1: sets the amount of images that shall be shown at the screen at once to 1
* use ImageMagicks 'convert' method to convert to xpm: $ convert input.jpg output.xpm
Have fun!
