Unfortunately at this moment is not possible to create frontend user that has permission to edit library.
If you want, you could execute the following steps to create a backend user without any permission except for managing libraries:
1. Create backend user without any roles (Administration -> Users -> Create a user)
2. Navigate to the library that you want this user can administrate (Content menu -> Images)
3. Click “Actions” menu
4. Select “Set Permissions”
5. For section “Update this library and manage its images” select button “Change” (If this button is not visible, check “Break inheritance” button)
6. Add new user that can administrate this library as check button “Add roles and users” -> In tab “Users” select the user that you want
7. Log in with the newest user -> the user can navigate only to Content menu “Images”.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items