How do I force an overwrite of local files on a git pull?

The scenario is the following:

• A team member is modifying the templates for a website we are working on
• They are adding some images to the images directory (but forgets to add them under source control)
• They are sending the images by mail, later, to me
• I'm adding the images under the source control and pushing them to GitHub together with other changes
• They cannot pull updates from GitHub because Git doesn't want to overwrite their files.

This is the error I'm getting:

  
error: Untracked working tree file 'public/images/icon.gif' would be overwritten by merge

How do I force Git to overwrite them? The person is a designer - usually, I resolve all the conflicts by hand, so the server has the most recent version that they just need to update on their computer.