Just saw it, a few clarifications. SPOILERS

Her ex-husband brought back the picture.

It appears that she and her son were in the car when William H. Macy's vehicle hit it. He may have been a drunken driver; that's not clear, but I think it's a good guess, especially considering Jen's rage at him.. Jen's character almost died herself, so the boy couldn't have been on a bike during the accident.

It's fuzzy to me - albeit I dove off a couple of times in the theater from working 80 hours this week.

The sequence after she seemingly OD'd - it took me a while to understand that she had OD'd, because I thought there were flash backs.

The keys?

Why wouldn't we see her (ex)husband at her bedside?

There's lots that was left off - perhaps out of artistic direction.

Not sure.

She was in a car accident which injured her and killed her son.

Her ex brought over the picture and his keys. He thought she needed the picture to start getting past it.

Although it is very hard to make clear what exactly happened, this is what I gathered from the flashbacks and movie altogether. The man was driving a car( he possibly was drunk), and hit her son who was riding his bike. Claire was probably very close to him at the time of the accident, seeing how she has injuries as well. Or the man driving (again possibly drunk) hit his car into Claire's car in which her son was a passenger. This would also explain her son dying and her having injuries. I did find it interesting that the woman who played her support group leader is married to the guy ( in real life) who played the man who caused the accident that took her son's life.