Also make sure that you are calling it from the right directory. From what i understand from your post, the first one (that doesn't show up) you call via going into the images folder and then finding the logo, whereas the second one you mention, you call it straight from the folder that your html page is in.
So you can name your jpg files with a png or even an arbitrary extension such as zzz and the browser should still show it as long as the server serves the correct content type, or the browser determines it is an image through other means - actually reading part of the file.
If you use firefox, you can use the firebug extension to track the HTTP requests recieved for a page. The image should be one of them. It will also tell you the HTTP response from the server for that image, it should give a HTTP status of 200. If it is something else, then there is a problem. Maybe the image is not readable by the server.