Twitter shows up pic's preview for a few tweets only


#1

I’ve noticed that only a very few photos are shown up with preview in twitter hashtags while the other are just with links and u need to click that pic’s link for preview
Do they use some kind of picture converter

The picture is uploaded straight to twitter and hosted by twitter only.
I thought it’d be a matter of more faviorites or retweets but it’s not actually.


#2