The Standardlogo theme allows the user to upload a logo via the regular Moodle interface and have it appear on the Site's FrontPage.
Add logo via front page files
To add your own homepage logo to this theme. Create a logo in a jpg or gif file. Make sure it is called logo.jpg or logo.gif.
Using the course administration block settings >Front page>Files and then upload the file into your Front Page files.
How to upload via site files
Alternatively put a file called logo.jpg or logo.gif in the main Moodle directory. (using FTP, for example).