There are several ways to start a file transfer. You can drag a file directly onto a Contact that is online. You can also right-click an online Contact and select "Send File". Lastly, you can select a Contact and press the "Send File" toolbar button (blue document icon). HGC-Skype then lets you send a file to other Skype users. The recipient will have to accept the transfer in order for it to begin. You can only send files to contacts who have authorized you.