One of my non-main accounts, which I happened to be working on today, just got the native Twitter images feature turned on.  

Here's a screen shot of the dialog that explains it.  

Here's Twitter's user docs. 

Of course I had to try uploading a mildly NSFW picture of Clark Kent checking out Lois and Lana with x-ray vision.  

And here's what the tweet looks like!  

Sorry if this offends, I think it's cute.