see attached image, but basically having the images on the right side makes it so hard to "read" a post with an image, going back and forth with your eyes to "get" the whole post. So much easier to just put the images between the up-/downvote buttons and the text post.
I don't know if there's a reason for that besides looking different from similar sites. If so, I don't think this is the way, it really puts a strain on your eyes and our brain is not used to absorb information in this way.
Or you can move the button to expand the image also to the left/right depending on where your thumbnail is.
That's basically what I liked most about the Relay app for Android - easily switch between left and right depending on how I hold my phone.