@pamela If it is GPLv2, and not AGPL, then yeah, you do not need to open source the flan images. That is, as long as you do not distribute the fork you added them to. As soon as you distribute, you either have to open source the images, or cease distributing.
Running the fork privately, with flan images added, is fine by the GPL (but not by the AGPL, and iirc not by GPLv3 either; but I'm not sure on this last one). No need to host modifications in this case.