Hi Matus!
I have the same problem liek echelon had. I created a simple project which includes a Human-Task and a script task.
My bpmn file has the following properties: Id: com.sample.hello; Name: HelloWorld; package: defaultPackage;
According to your suggestion I should name my image file as com.sample.hello-image.png, because com.sample.hello is the process id.
In this video screencast (http://people.redhat.com/kverlaen/jBPM5-guvnor-integration.swf) krisv named the process image file as Hello-image.png (at 2:20 you can see that Hello is actually the Process Name no the Id).
I tried both suggestions but neither worked.
Currently I have three items in the default package (in drools-guvnor): sample.bpmn, HelloWorld-image.png and com.sample.hello-image.png but the process image won't shop in in jBPM console.
Console version: 2.3.5.Final
Guvnor version: Version 5.4.0-SNAPSHOT