Alloy theme definition interface allows to assign a customized image for each signature. It is possible to choose an image from a list of predefined images. But, I am wondering if it is possible to add my own image to this list?!
The alloy source code is available and you may edit it to your needs. The source code is included in the jar you use to run the Alloy Analyzer.
There is no built-in customization to do this.