TOP left of Frontpage go to Submit content/Logo

That is here
http://www.userlogos.org/node/add/logo

Here is the browse and upload part of that page
Do you not have that ?
Having filled out the different parts of the form then go to the box seen here
Browse first to your Logo on the hard disk, and once that information loaded click on Upload

That should be sufficient

It is worth noting that the Logos uploaded on this site should conform to the aspect ration of the other logos