To enter and exit thumbnail mode, use the button Thumbnail on the toolbar. Thumbnail mode is intended for sorting and renaming image files by displaying the actual image instead of the small icon in the list.
You can use the settings dialog to specify the size of your thumbnails.
Next to the Thumbnails button there is a View Style selection button. The button can be used to change how the files will be displayed in the list. Some views are more suited for sorting image files than others but displays less details.
Be aware that extracting thumbnails from all the files in the list may take some time and stress your computer hardware. Using too big a list in thumbnail mode can make Advanced Renamer and your computer slow.