Hi Chris,
old iPads as picture frames is a good idea :) I also got one laying around useless.
What you describe is a front-end problem in the old browser which is unaware of newer Javascript features. Unfortunately Safari is dusty old on these devices. There are two things you can do I think:
1.
Try an older version of PicApport which uses an old Webinterface (but then the complete program is old and lacks of all the good features)
2
Use the API of PicApport, organise your photos with the latest version and make your own "Viewer" component (Website which loads and shows the pictures/slideshows but with way less features in the fronted)
So, I would recommend the second approach. Are you a programmer? - Maybe there exists a lightweight fronted anywhere?
Regards!
Michael