I put a relative path to my image in to "login image"

Hi,

I put a relative path to my image in to "login image" field, the image shows in ultimate branding admin (images tab) and on the login form too, but i have this ugly warning in both sides:

Warning: getimagesize(/wp-content/themes/mytheme/assets/img/my-custom-image.png): failed to open stream: No such file or directory in /home/myuser/public_html/wp-content/plugins/ultimate-branding/ultimate-branding-files/modules/login-image.php on line 246

if i put the complete url i don't have this error, but i want make the image relative because we have various environments to the site (development/quality/production) and the image must be see in all environments without need change the site_url.

Any clue? how i can do this? thanks in advance