Returns the default name of the image resource for a certain case.
It will first look for an image resource which matches the case
specifically, before resorting to an image resource which matches the CMMN
1.0 xml file resource.
Example: if the deployment contains a CMMN 1.0 xml resource called
'abc.cmmn' containing only one case with key 'myProcess', then
this method will look for an image resources called 'abc.myProcess.png'
(or .jpg, or .gif, etc.) or 'abc.png' if the previous one wasn't found.
Example 2: if the deployment contains a CMMN 1.0 xml resource called
'abc.cmmn' containing three cases (with keys a, b and c),
then this method will first look for an image resource called 'abc.a.png'
before looking for 'abc.png' (likewise for b and c).
Note that if abc.a.png, abc.b.png and abc.c.png don't exist, all
casees will have the same image: abc.png.