It took me awhile to find the old bookmarklet solution, now that WordPress has upgraded the PressThis functionality.
Here’s a copypasta from (and a link to) the WordPress codex page:
Using this bookmarklet to publish posts does not pass the $_POST[‘post_type’] variable with the value of “post”.
The bookmark calls http://example.com/wp-admin/press-this.php?u=&t=&s=&i=
u = the url of the current page
t = the title of the current page
s = the text selection from the current page
i = url of an image file
Edit the bookmark within your browser to change the values passed if necessary.