The dialog box was
[recently redesigned](http://meta.stackexchange.com/a/168763/1438),
but the functionality hasn't really changed. If you have an image in
your clipboard, you can use Ctrl-G to bring up the Imgur dialog box
and paste the image using your OS's paste-from-clipboard keyboard
command (often Ctrl-V or Command-V). No need to use the mouse or the
file manager.

If you already have an image hosted somewhere, you can also use
Markdown to link to the URL:

    ![New Imgur uploader](https://i.sstatic.net/qag6ht.png)

![New Imgur uploader](https://i.sstatic.net/qag6ht.png)

Malabarba has a rather fine
[Stack Exchange mode](https://github.com/vermiculus/sx.el) for Emacs,
but it does not seem to support image uploads just yet. Perhaps it
could provide some relief to your hotkey annoyance when composing posts?