If the selected file in the Detailed Information window is a picture file or if a mp3 file contains a picture, then a thumbnail of the picture will be displayed in this panel. The display can be disabled in the general options.
The thumbnail can be resized by moving the sash located in the blank area between the Thumbnail label and the preview list. Just move the mouse over the sash area and a double vertical arrow will appear. Left click on the arrow and drag to resize the thumbnail panel; the thumbnail will change size with the panel.