i fixed a deprecation issue that had rendered the plugin useless for me.
split() is apparently deprecated in 5.3 but can be successfully replaced with explode() except that the string "\." is not valid instead it must be "." - took me a bit to figure that out.
hope this helps others with a similar issue problem.
* the file i edited is timthumb.php under wordpress-popular-posts/scripts/. just search for split() and switch:
$frags = split( "\.", $_REQUEST['src'] );
$frags = explode( ".", $_REQUEST['src'] );