In what area are you and what did you try to do?
This image seems to be added in your define page, meaning you can edit from admin->Tools->Define Pages Editor->define_main_page.php
I have just added a style tag and it worked on my end:
Code:
<img style="max-width:100%" src="images/large/cover1.jpg" alt="Stitch N Frame">
Keep in mind this is not the best approach in long term - the file is simply too big. The original image is 1800x1039 and 470 kB. You will never display it larger than 1600px (your container size) so you could resize the image, and then you could optimize it (tinypng or similar). I got it down to 249 kB.
But... Why are you using template_default, is something wrong? Why not Responsive Classic, or even better the NEW responsive classic or ZCA Bootstrap templates?