Adicionar imagem

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How can i add a image, a 50 x 50px square, at the end of the floating box??

Thank you

You can use the socialfblog_buttons filter.

Example:

function your_function_name( $html ) {
    $html = $html . 'YOUR IMAGEM URL HERE';

    return $html;
}

add_filter( 'socialfblog_buttons', 'your_function_name' );

Use this in your theme functions.php.