The file list can either show the default icon for an image, or a thumbnail image of the file. Thumbnails can be created for image files and are stored in a folder called .Thumbnails in the same folder as the image.
There are two ways to make thumbnails. Setting the 'Make thumbnail when image is viewed' option in the preferences makes ImageBrowser make a thumbnail when the image is shown in the image view, if one doesn't already exist.
Alternatively, you can create thumbnails for all files in a folder using the commands in the Thumbnails menu. You can also create thumbnails for all files in sub-folders of the current folder, or just for selected files.
The Thumbnails menu also has commands for deleting thumbnails.