You can add parameters inside the [jwp=425,344] like this [jwp=width,height,image] and add them in the code.
To add preview image use this code:
SMF 2
/Sources/Subs-Editor.php
array(
'image' => 'jwp',
'code' => 'jwp',
'before' => '[jwp=width,height,image]',
'after' => '[/jwp]',
'description' => $txt['jwplayer']
),
/Sources/Subs.php
array(
'tag' => 'jwp',
'type' => 'unparsed_commas_content',
'content' => '<object type="application/x-shockwave-flash" data="' . $settings['default_theme_url'] . '/player.swf" width="$2" height="$3"><param name="movie" value="' . $settings['default_theme_url'] . '/player.swf" /><param name="AllowFullScreen" value="true" /><param name="AllowScriptAccess" value="always" /><param name="WMode" value="transparent" /><param name="FlashVars" value="file=$1&image=$4" />$1</object>',
'validate' => create_function('&$tag, &$data, $disabled', '
if (isset($disabled[\'url\']))
$tag[\'content\'] = \'$1\';
elseif (strpos($data[0], \'http://\') !== 0 && strpos($data[0], \'https://\') !== 0)
$data[0] = \'http://\' . $data[0];
'),
'disabled_content' => '<a href="$1" target="_blank" class="new_win">$1</a>',
),
SMF 1
/Themes/default/Post.template.php
'jwp' => array('code' => 'jwp', 'before' => '[jwp=width,height,image]', 'after' => '[/jwp]', 'description' => $txt['jwplayer']),
/Sources/Subs.php
array(
'tag' => 'jwp',
'type' => 'unparsed_commas_content',
'content' => '<object type="application/x-shockwave-flash" data="' . $settings['default_theme_url'] . '/player.swf" width="$2" height="$3"><param name="movie" value="' . $settings['default_theme_url'] . '/player.swf" /><param name="AllowFullScreen" value="true" /><param name="AllowScriptAccess" value="always" /><param name="WMode" value="transparent" /><param name="FlashVars" value="file=$1&image=$4" />$1</object>',
'validate' => create_function('&$tag, &$data, $disabled', '
if (isset($disabled[\'url\']))
$tag[\'content\'] = \'$1\';
elseif (strpos($data[0], \'http://\') !== 0 && strpos($data[0], \'https://\') !== 0)
$data[0] = \'http://\' . $data[0];
'),
'disabled_content' => '<a href="$1" target="_blank">$1</a>',
),