The ++ (as in your ++resource++assets) introduces a so called namespace (in your case with namespace type name resource and namespace name assets). Namespace lookup is supported by [un]restrictedTraverse. This implies, that you can e.g. use obj.unrestrictedTraverse("++resource++assets/images/ClassIcons/SoftwareLicence.png") (where obj is a Zope web object, typically your context object).
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