First, thanks to Baptiste for reporting a developer notes issued by Skype, and explaining how to use the iSight with its maximal resolution (640x480), while Skype is originally locked to 320x240.
Then, you will need to have at least the version 220.127.116.11 installed on your computer.
You then need to edit the file entitled "config.xml" and located in ~/Library/Application Support/Skype/YourSkypeAccount/
Replace the following code
Save the file, then relaunch Skype. You will of course need a fast upload internet connection, and this higher resolution will give the opportunity for your contacts to better enjoy your nice face, so smile