Heading and subheading text is updated direclty in the Theme Editor. To customize form elements like the placeholder text and submit button, use the Language Editor and modify under Newsletter:
Choose Edit Languages from the theme's actions button
Modify form elements under Newsletter
Changes in the Language Editor will apply to all Newsletter forms on your site
Image with newsletter form
To create a two column Newsletter sign-up with an image, use Image with text and choose the newsletter content block.
