For this theme specifically, the #feature is the main image at the top of the page and the #extraContainer are the other images as you say.
extraContainer is actually “ExtraContent” areas that in this theme display the background image. After placing your image into this area you can use the code on this page http://michaeldaviddesign.com/themes/endeavor/tutorial/index.html (myExtraContent) to display the text/images etc over the top of that image. (Or you can use the extracontent stack)