This is a Master User feature. If you do not have access to Account Settings you can not update the portal description.
The Portal Sign In page can be customized in a few different ways; logo. background image, description. What do you call your Image Relay portal internally; the brand center? the image bank? Perhaps add that description here, or uncheck this box and let your logo stand by itself on the sign in page?
Go to Account Settings and look for Display on Portal Sign In. Text entered here will display on the sign in page of your portal beneath your logo.
Then scroll down to the bottom of the page. You may Preview or Save.