User permissions: edit existing elements only

Hi! Upfront Builder is really cool!

So I am attempting to figure out how to give my subdomain creators access to the specific elements I have created on the Upfront theme allotted to them. I want them to be able to:

1. Change the header picture (but keep the same element dimensions)
2. Modify the text in the provided text box (but not remove or change the element otherwise)

I see User Permissions allows them to either modify pages or not, and delineates between the front page and other pages. But I don't see an option to cover the points I have expressed. If there is no User Permissions option, is there some CSS shortcode I could place in the box to make it's contents directly editable?

Thank you!