It seems to me that the first pre-requisite for analyzing the print on the bathmat is an accurate outline of its shape. Drawing it by hand is too subjective, in my opinion, but extracting the shape with a graphics program is a slightly tricky process.
I used Gimp to create an outline that I believe is as close to being accurate as we can get, and I have posted it at:
http://www.friendsofamanda.org/footprint_outline.jpg
This is a composite of several iterations to show the method I used. On the left is the Crimescope photo of the footprint from a pdf of the police forensics report, enlarged to about twice the native resolution of the pdf. It is the basis of my outline.
Next is the image I got when I further enlarged the crimescope photo, and then selected pixels by color value with a 25% tolerance on the initial selection, reduced to a 10% tolerance to pick up the edges. I used an edge detect filter to reduce the image to a grayscale negative, which I inverted to produce the first iteration shown in my composite.
Next, I filled the area around the footprint in white, and filled the inside of the footprint in gray, using a high enough tolerance to produce the solid mass that is the second iteration in my composite.
Finally, I used the edge detection filter again and inverted the result to produce a dark line with a white background.