First step is to navigate to your ACP>Templates & Style>Themes>your current theme>global.css and add this code:
Code (Text):
Next we need to assign the class we just added (shake) to the logo. Navigate to ACP>Templates & Style>Templates>your current theme>Header Templates>header and find the reference to your logo and add class="shake" to that line. Example below:
PHP Code:
[URL="http://yoursite.com"][IMG]{$theme['logo']}[/IMG]settings['bbname']}" title="{$mybb->settings['bbname']}" />[/URL]
Save and refresh your forum to test effect. Note: the shake effect can be add to most images on forum if you want, just experiment with adding the class="shake".