Dynamically named attributes cannot be automatically exposed on the canvas and used by other transformers.
You can write them to a format if you use the SchemaSetter custom transformer and a writer set to Dynamic Schema Definition.
Personally I wouldn't use a loop at all, just a Cloner set to Max_Images, and build the attribute name from the _copynum attribute. This would also avoid the formatting issues of 2.0, 3.0, though that could be taken care of using the StringFormatter.
It doesn't look like you are exposing the new attribute names?
Like Takashi I cannot access your workspace, but in situations where @Value(attribute_name) hasn't worked in the past (double check that it's a capital V), I've had success with FMEFunctionCaller using the @SupplyAttributes(attrName,attrValue) function.