There is an old thread by this name that attempts to discuss the subject. However the thread has been conveniently truncated to remove the information you really need (draw your own conclusions).
For the later versions of IDM the path to place images in is:
/opt/netiq/idm/apps/tomcat/webapps/IDMProv/images/
(This also appears to not be documented)

For the 4.0.2 UA I'm not sure where to stick them however. It was never added to the docs and as previously mentioned the answer has been deleted from the forums.
Given that there are still a lot of folks using the 4.0.2 stack I thought it would be useful to document this here.

Does anyone know the path for 4.0.2 to place the images in? It is different from the engine context so it's not the manual task driver path. I suspect it is similar to the above but I don't have a great way to test this.