When a user only right is 'Use Gallery' they can't delete images when they click
the delete icon.

If you use 'Require userpermissions to edit subgalleries' and only select that
user for a certain subgallery (using the editors list). They can only add and
modify their own images in the subgallery. But they can't delete files in their
own subgallery.

Gallery options:
[CHECKED]   Require userpermissions to edit subgalleries:

Backend groups permission: 
[unchecked]  Gallery - Add subgalleries			
[unchecked]  Gallery - Delete subgalleries			
[unchecked]  Gallery - Edit all galleries
[CHECKED]   Use Gallery

When 'Edit all galleries' is selected. the limited user can also delete other
subgalleries, but that is too much. They only need to be able to delete within
their own subgallery.