@Entity
public class Image {
    ...
    @OneToMany(mappedBy = "image")
    @MapKey(name  = "property.name")
    private Map<String, PropertyValue> properties = new Hash Map<String, PropertyValue> ();
    ....
}

@Entity
public class Property {
    ...
    @Basic
    private String name;   
    ....
}

@Entity
@IdClass(PropertyValuePk.class)
public class PropertyValue {
    ...
    @Id
    private Property property;

    @Id
    private Image image;

    @Basic
     private String value;   
    ....
}

@Embenddable
public class PropertyValuePk {
     ....
     @ManyToOne
     private Property property;

     @ManyToOne
     private Image image;
    ....
}