Hello Sergio, did you get memory errors or why do you ask about that? The first-image-rule does need very little memory. A user of the plugin reported he set over 200 rules with no problems (until a database error occured, not a memory error).
The “Illegal string offset” warning has nothing to do with memory. Indeed that is a rare error. If you would send me the login credentials to your WP site I would take a look. Please use firstname.lastname@example.org.