Yes, you can add a logo already defined in the Organization or Event as the Organization Logo or the Event Logo. If you don’t already have an image defined for the Event, go into the Event Settings tab | Logo sub-tab and click on “Set Logo…”  Here you can input a URL to a website where your Logo is located, or you can click on Select to upload a new image from your computer.  Then you can use the variable [EVENT_LOGO] to place it on your Signup Sheet.
To add the Organization or Event Logo, go into the Signup Sheet Design tab and click on the Top Instructions button.  Then, in the Variables menu, select[ ORGANIZATION_LOGO] for the Organization Logo, or [EVENT_LOGO] for the Event Logo.