I do not understand your query. Please give a code snippet of how you are "logging in" to a site you have not written yet??? When you say API do you mean URL?
On the naming of web pages, only the home page needs to be standard, usually "index.html". So the full url is, say: "http://www.mysite.com/index.html". Other web pages are usually put in the same folder on the server so their relative address is just the page name. E.g. "AboutUs.html". So in index.html (or any other web page in the same folder) the link: <a href="AboutUs.html">About Us</a> will display the "About Us" page.
Incidentally, it is generally best to put graphic files in a separate sub-folder, often called "graphics". So the code <img src="graphics/mypiccie.jpg" alt="my picture" width="100" height="150"> will display the picture. Note: The width and height are optional, but should be given as that minimises the "jumping around" of the page as it loads.