If you can see pictures in other sections of your portal, it seems mapping is ok. just make sure that your logo is in jpeg or gif formats. if you use png file you must make some changes in mapping and config files.
so, if code below doesn't work
<img src='<%= ResolveUrl("~/Admin/Images/logo.gif") %>' alt="my logo" style="border: 0;" />
try to pinpoint logo image by using HTML.
Good luck!