How to make a fix backward compatible with old data?

[Salima Begum]

Hi all,

We used to allow uploading images to a market place Ad and when a user doesn't upload one of the images we present a dummy image in the UI as well as saving the same dummy image in the database. 

Now we added a fix for not saving dummy image and just saving whatever user uploads. That means when a user uploads 2 out of 3 images, we just upload 2 images and present them in the UI without any dummy image.

So this creates new problem now because the old Ads display dummy images in UI and newly posted Ads don't show dummy images because of the fix that was added. I would like to know how to make the fix backward compatible to the old data?

Thanks

~Salima