Is there any way to turn off the 'Default Section Image' as we do not want to display any images at all?

Go to 'Settings | Site Options | Layouts'.

In the 'Sections' area, against 'Section Link Layout' use the drop down to select an appropriate layout. Here you can also use the option to create a layout based on an existing one. You can then edit the layout in Design | Library | Layouts | Section Links . Right Click on the layout and select Edit to make changes.