You can change this via the Account setting page.
Click on your username to open the drop-down list, on which you can see the 'Account' option
Then click on the 'change' hyper link beneath the profile picture box.
Browse and select a new profile picture.
Sign into your YouTube account and
click the 'Account' link at the top
right hand corner of the page
Click the large blue 'My Account'
link in the top left hand corner of
In the left hand navigation column,
click Profile Setup.
Click the 'Change Picture' button and
select the file that you'd like to
display as your thumbnail profile
icon on your channel (you can either
select an image or a still frame from
one of your videos).
Don't forget to click the 'Save Changes' button to save the update
you just made to your channel.