Custom logo for each organic group
I write this module to render custom logo for each Organic Group:

function og_logo_block($op = 'list', $delta = 0, $edit = array()) {
    if ($op == "list") {
        $block = array();
        $block[0]["info"] = t('OG Logo Block');
        return $block;
    } else if ($op == 'view') {
        global $theme;
        global $node;
        $block['subject'] = '';
        $settings = theme_get_settings($theme);
        if (($node = og_get_group_context()) &amp;&amp; node_access('view', $node)) {
            $block['content'] = l('&lt;img alt="' . $node-&gt;title . '" title="' . $node-&gt;title . '" src="' . base_path() . $node-&gt;field_logo[0]['filepath'] . '" /&gt;', "node/$node-&gt;nid", array('html'=&gt;true));
        } else if ($settings['default_logo']) {
            $block['content'] = '&lt;img src="' . base_path() . 'sites/all/themes/' . $theme . '/logo.png" alt="this is logo" /&gt;';
        } else {
            $block['content'] = '&lt;img src="' . base_path() . $settings['logo_path'] . '" alt="this is logo" /&gt;';
        }
    }
    return $block;
}