Modify Image Source With The_Content Filter?

Let’s give it a try:

$str="<img src="http://myserver.mydomain/picture.png" />";
$pattern ='#<img src="http://.*/(.*)" />#';
$replace="<img src="http://http://i0.wp.com/$1?quality=70&strip=all />"";
$result = preg_replace($pattern, $replace, $str);

The $pattern is quite simple and very specific. You might need to generalize it a bit more but it should show you the direction to go. E.g. it takes no care of possible whitespace. A first attempt to take care of this:

$pattern ='#<\s*img\s*src\s*=\s*"http://myserver.mydomain/(.*)"\s*/\s*>#';