Okay just going to put all the details here and tweet the developer (who is genuinely helpful) I’ve seen active on the forum.
Tweet a URL to a gallery card, will only load a summary card without any pictures in the news feed, however it will load a full 4 images if you view the details of the card…
example url: stolen-bikes.co.uk
example feed: https://twitter.com/mystolenbikes
example details: https://twitter.com/MyStolenBikes/status/346606664767647744
This is in FireFox 19… however try the very same thing in IE 8 and it appears to work fine… At this point I’m wondering if it’s browser related… (it could be my cache but I’ve tried clearing it).