Take a picture of your face. Edit in photoshop, Artweaver, GIMP, (even MS Paint can work) to cut out whatever parts (neck, background) etc you don't want, leaving the remaining background transparent - then save as LockSliderThumb.png. (This is case sensitive.)

Find your respective theme folder: /opt/usr/share/themes/ or /home/opt/usr/share/themes - depending on what version the theme is and when it came out, it'll get saved to different places on the N900. Copy to [your theme's folder]/images. To copy, either use x-term as root, "cp [where you actually have file] [where it needs to go]", or use a file manager that has root access.

There's no program that will recognize your face, cut out the rest, and do it for you, though.

Edit: Forgot a somewhat important note - resize the image to 148 x 94 pixels. (I think that's what it is.)