The one thing that people I work with mess up most often is inserting thumbnails and linking them to larger size images. I'm sure a very common way of working is to insert a thumbnail and have a larger version open in Lightbox.
When adding an image it would be useful if the Link URL field could have extra buttons underneath in addition to the "None", "File URL" and "Post URL" buttons. Maybe "Medium image" and "Large image".
I know it's possible for them manually to enter the URL of another image but this would make it easier.