It will not likely be 72 dpi. The best web design idea is to design for the web visitor's smallest likely screen size, like 640x480 pixels, or maybe 800x600 pixels at most. Your own screen's size is not important to anyone else. It should be noted that printing from a web browser is a special case . Browsers are video screen oriented, and also will not look at image's scaled dpi.
Video shows only pixels on the screen.
But browsers have to deal with paper too. It's a long story, mostly a distraction, but it is pertinent here since this is a web site page, and you are using a browser. So to prevent additional confusion, it should be mentioned that in Windows, Netscape and MS IE browsers when printing full web pages, will scale embedded web page images to 96 dpi size for printing. This dummy 96 dpi value will better match the proportion of screen image size to printed text size. It is really only about text.