If you set a logo for an event, that Event’s logo will appear on Facebook.  To do this, sign in as an Administrator and select the relevant event.  Go to the Settings tab | Logo sub-tab.  Press Set Logo, then either type in a link to a logo hosted on your own website, or press Select to upload an image file from your computer.  Press Save after the logo is set.
If you set a logo for the Organization and there is no logo for the Event, then the Organization’s logo will appear on Facebook.
IMPORTANT: Sometimes it can take more than 24 hours for a logo that you just set to appear when you access your ivolunteer.com Home Page or on an ivolunteer.com Event page from Facebook.