I don't have a slice but I have used it before and I imagine it would be the same are using another diecutting machine where you can't exactly see what is happening.
let's say you want to cut a circle from your photo. I would cut a circle out of paper using the Slice to see exactly where it will cut. Then I would use that cutout to help determine where on the mat to place the photo to get it to cut the circle from where you want. Definitely use copies of your pics until you get what you want. hth