As it turned out, I simply forgot putting the map on the DbContext:
modelBuilder.Configurations.Add(new CustomerImageMap());
That said, the composite Id still not being populated on the $metadata this way. And so using data annotaion this is the metadata that is generated:
<EntityType Name="CustomerImage">
<Key>
<PropertyRef Name="CustomerId"/>
<PropertyRef Name="ImageId"/>
</Key>
<Property Name="CustomerId" Type="Edm.Int32" Nullable="false"/>
<Property Name="ImageId" Type="Edm.Int32" Nullable="false"/>
<Property Name="LastUpdated" Type="Edm.DateTime"/>
<NavigationProperty Name="Customer" Relationship="EasyBizy.Entities.Models.EasyBizy_Entities_Models_CustomerImage_Customer_EasyBizy_Entities_Models_Customer_CustomerPartner" ToRole="Customer" FromRole="CustomerPartner"/>
<NavigationProperty Name="Image" Relationship="EasyBizy.Entities.Models.EasyBizy_Entities_Models_CustomerImage_Image_EasyBizy_Entities_Models_Image_ImagePartner" ToRole="Image" FromRole="ImagePartner"/>
</EntityType>
However, if using fluent API instead of data annotation, the key part is not being generated at all. Why?