For Activity page

Copy views\default\river\elements\image.php from core to your own theme/plugin:

\mod\your_plugin_name\views\default\river\elements\image.php

Change:

if (elgg_in_context('widgets')) {
    echo elgg_view_entity_icon($subject, 'tiny');
} else {
    echo elgg_view_entity_icon($subject, 'small'); //Arrays: large | medium | small | tiny | topbar
}

For members page

Use these hooks in your own theme/plugin (start.php):

elgg_unregister_page_handler('members', 'members_page_handler');

elgg_register_page_handler('members', 'my_members_page_handler');

function my_members_page_handler($page) {
    $base = elgg_get_plugins_path() . 'your_plugin_name/pages/members';

    if (empty($page[0])) {
        $page[0] = 'newest';
    }

    $vars = array();
    $vars['page'] = $page[0];

    if ($page[0] == 'search') {
        require_once "$base/search.php";
    } else {
        require_once "$base/index.php";
    }
    return true;
}

Copy index.php and search.php from

\mod\members\pages\members\

to:

\mod\your_plugin_name\pages\members\

Change string in index.php:

$params = array(
    'options' => array('type' => 'user', 'size' => 'large', 'full_view' => false),
);