User profile pictures disappearing - OU migration in AD

I'm not expert on the synchronisation of user profiles between AD and SP, so I'd be hugely grateful if somebody could clarify and expand on the following - User profile images for users are disappearing from my organisations SP farm. This only affects accounts that have migrated to a differing OU in within AD (these users stay in the same domain). Could anyone explain why that occurs? Is it because SP treats each migrated profile as a new profile entirely and doesn't carry the picture across? or is it some reason entirely different? Going forward, all users within the organisation are being migrated OU's within AD. I am hoping there is a way this migration can be implemented without losing all the user profile pictures for the entire organisation. Again, any suggestions welcome?