Hi Meagan,

Currently this feature only available for normal status update and comment stream type. We do however have been trying to implement this feature during the development of 2.0 but editing a rich media post such as photos stream is extremely complicated because it's not just about throwing a text editor form there.

For instance, when you post a photo on the stream, we need to rewrite the entire form to upload photos to be able to "edit". In other words, previously uploaded photos needs to be there in the editor and it involves a very tricky process and possible complication such as deleted photos, deletion of photos from albums, privacy of the albums, previously shared photos stream by other users and many more! And here we only talking about photos stream and there are multiple instance of streams type such as videos stream and file stream that having different kind of complications.

It may sound easy to just simply add the "edit" button but there are a million reasons behind not implementing them. Hope you understand.