simple maintenance mode with image

I found this code to put in my .htaccess file (codeigniter 3.0.2) to put my application into maintenance mode except for my IP address so that I can continue working on it:

PHP Code:

Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REMOTE_ADDR} !^75\.75\.75\.75$
RewriteCond %{REQUEST_URI} !construction\.html
RewriteRule ^(.*)$ /testRES/construction.html [R=302,L] 


It works well, but my under construction page is basically a background image. I tried putting the image in the root of the application, and I tried putting it in the assets/images folder and I tried all sorts of different versions of the path to the image, but I can't get it to display.

What is the best location for the image, and what would be the path to that image from the construction.html file that is in the root of the application folder?