When you use your own pet image in Pilktogram, it should be png with a transparent alpha background. It is best if you do this using a photo editing program such as GIMP or Photoshop, but the Pilktogram Pet Picture Part Preparer is a quick way of producing the necessary image file, if your pet is sufficiently distinct from the background.
In May I wanted to review C++. I thought about a few project possibilities and in my research found the OpenCV image processing library, and the QT cross-platform C++ application and UI framework. OpenCV has an implementation of the Grab Cut algorithm, which is used to separate a foreground object from its background.
So I give you the PPPPP, available for Mac OS X (10.6 or later, tested on 10.7 or later), and Windows (Tested on Windows 7 and XP) [I don't do windows, but QT made it easy enough)]. I can also provide binaries for some Linux versions.