Thanks!
I'm assuming you already have the SB Widget installed so yes, all you need to do is change the Layer.png file (for background) and Girl.png (for the picture in the middle). The latter takes a couple of extra steps;
Basically, I took a pic of my own and cropped it to 267x636. I then opened the Girl.png file in Pixlr.com (online Photoshop tool, although if you have Photoshop, you can use that too). After you've got the Girl.png file open, open your pic as a new layer and position it directly over top of the original image. Then delete the original layer and save as Girl.png, then replace the original with the newly created file.
For the LS, I created a whole series of Fringe backgrounds (because I'm a fanatic) and whichever one I want, I rename as Layer.png. I also modified the LockBackground.html file to position the barcode font lower and move the weather widget to the right side instead of centered. If you want, I can tell you what you need to change but I highly suggest you do this on a copy of the file so that if you bugger it up beyond repair, you can easily revert to the original file.
Of course, I've since made more changes (albeit minor) to my springboard:
That's a photo of my little guy BTW...he's awesome!