Well, first thing first you need to learn how to do login page in asp.net, which is not that hard. For starters see this MSDN guide.
How to: Create an ASP.NET Login Page[
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Once you have you login page up and running, it is not that hard to add an image to it. In your user database (table) add a column to hold either the image itself or a path to where is the image is located. From there you can load that image in any page you like.