It looks like Excel has made copy-as-image default behaviour for the Ctrl+C hotkey for some reason. The new editor supports copying & pasting multiple images and files as attachment from the clipboard, so it's just doing it when you paste the excel data. I'm afraid you can only manually delete the image in the editor after pasting the table data for the time being. We will do something about it.
There seems to be a bug in the bbcode plugin for the table in the old site. Now the compatibility issues related to the tables between the new and old site has been resolved, so you can probably start pasting tables in the new site?
