Lots of people are reporting problems with it recently. I know that the team are working on it as fast as they can.
An easy way to find out if it is the TinyMCE is to turn it off and try writing a new post. If it works without TinyMCE then I would suggest you send a feedback from your dashboard letting the developers know.
I know that the “insert” on the image inserts at the last cursor position in the editor, I assume that you have the cursor set at the appropriate insert point?