You can try:

$element->outertext = '';

To illustrate:

php> $html = new simple_html_dom();
php> $html->load_file('t.html');
php> echo $html->outertext;
<html> <head> </head> <body> </body> </html>
php> echo $html->find('img', 0)->outertext;

php> $html->find('img', 0)->outertext = '';
php> echo $html->outertext;
<html> <head> </head> <body> </body> </html>
php> echo $html->save();
<html> <head> </head> <body> </body> </html>

Hope that helps.
nJoy =)