You could probably update the file: webassist/file_manipulation/helperphp.php
On line 1021 you should find:
imagecopyresampled($tmp,$src,0,0,0,0,$newWidth,$newHeight,$width,$height);
update that to:
imagecopyresampled($tmp,$src,round(($width-$newWidth)/2),round(($height-$newHeight)/2),0,0,$newWidth,$newHeight,$width,$height);