On the homepage and the browse pages, folders/access levels or categories are displayed with a generic folder icon; we recommend, where possible, to show a relevant thumbnail image instead.

Navigate to Admin > Folders/Categories and click edit next to one of the folders; you will see an 'Add image' field - simply use this to specify a thumbnail for that category. 

If you then navigate back to browse you should be able to see that that category is now shown next to the thumbnail image you selected.

If you are replacing an image and do not see the updated version this is usually a browser cache issue.
In this case, try 'hard refreshing' your browser using the hotkey "CTRL+F5" (Windows) or "CMD+R" (Mac).